Betterbird 115.16.1-bb34 + 128.4.3 ESR

115.6.0-bb21 "latest build (4)" (23 December 2023, Build-ID 2023-12-23*, English on Windows and Linux only)
This version is not offered as an update. Changes will ship in the next release.

Fixed in this version:

NEW - Column indicators for secondary search and icon columns - Bug 323067
CHANGED - Regular expression matching in search terms now using ICU's regular expression matching rather than std::regex
CHANGED - Enhancements to the splitters in the compose window, can now all be operated via the keyboard
FIXED - A sound was sometimes played for new feed messages even if disabled with pref mail.feed.play_sound. New prefs mail.feed.play_sound.type and mail.feed.play_sound.url to configure feed notification sounds if desired - Bug 1871493
Various minor fixes that Thunderbird isn't shipping yet:

Bug 1868094 Bug 1868816 Bug 1870619 Bug 1870720 Bug 1870809 Bug 1870873
115.6.0-bb21 (19 December 2023)
Based on Thunderbird 115.6.0, release notes.

Fixed in this version:

NEW - Account colours part 2 and 3: Message list (unified folders only) and Write/compose window
FIXED - Crash when entering an invalid regular expression into a filter search term
FIXED - Phantom rows and search problems relating to too many folders being searched. Should now work with up to 6144 folders. On Linux, increase ulimit -n according to the number of folders in the profile.

Fixed in Betterbird, not fixed in Thunderbird 115.6.0 yet:

Fixes provided by the Betterbird project and integrated into Thunderbird to be released later

FIXED - After restart, unread/total message counts not shown for collapsed servers in the folder tree, now also working for favourite and unread modes (fixed in 115.5.0) - Bug 1862774
FIXED - Window/tab closed when downloading rest of POP3 message when "close on move/delete" option was selected (fixed in 115.4.2) - Bug 1861886
Various minor fixes that Thunderbird isn't shipping yet:

Bug 522768 Bug 1799368 Bug 1812726 Bug 1823024 Bug 1837152 Bug 1846054 Bug 1862519 Bug 1862521 Bug 1864338 Bug 1866819 Bug 1867407 Bug 1867532 Bug 1868079 Bug 1868474 Bug 1868718 Bug 1868794 Bug 1868974 Bug 1869557 Bug 1870817
Features and fixes exclusive to Betterbird (new in the 115 series):

NEW - End-of-year Donation campaign: Betterbird will ask for a donation. This is only once per profile per year. (fixed in 115.5.2)
NEW - Account colours part 1: Account Manager and folder pane. (fixed in 115.5.2). Note: Color black (#000000) is interpreted as "no colour". The chosen colour is applied with 20% opacity.
NEW - More layouts: "Wide thread" (thread pane wide on top, folder and preview pane below), "Stacked" (folder pane and thread pane both on the left) (fixed in 115.5.2)
NEW - Attachment list on top in Write (compose) window
NEW - Make tabs session restore optional. Option to restore no tabs, system tabs or all tabs. (fixed in 115.5.2)- Bug 506526
CHANGED - Thread pane can now be collapsed to the height of one row (fixed in 115.5.2) - Bug 1855611
FIXED - Minimise to tray not working on Linux if attempted before tray icon shows for the first time (fixed in 115.5.2) - GitHub #227
NEW - Quick filter and folder search in S/MIME encrypted messages (fixed in 115.5.1)
FIXED - Global search in S/MIME encrypted messages not working (fixed in 115.5.1)
FIXED - Problem with manually sorting (ordering) IMAP folders in folder pane (fixed in 115.5.1)
FIXED - Empty "phantom" content tabs appearing after startup-up (fixed in 115.5.1)
RESTORED - Windows system notifications (fixed in 115.5.0)
NEW - Ability to manually sort (order) folders in folder pane (most of the work by the Thunderbird team, adjustments by Betterbird) (fixed in 115.5.0) - Bug 1846550, Bug 1864510, Bug 1864582
NEW - Introduced pref betterbird.update.checkIntervalDays to configure time between update checks (in days) (fixed in 115.5.0)
FIXED - Turning multi-line view on/off wasn't immediately reflected in the thread pane/message list (fixed in 115.5.0)
FIXED - Performance issue when using search/virtual folders (fixed in 115.5.0) - Bug 1864247
FIXED - Checking for add-on updates hung at "Updating add-ons" under some circumstances (fixed in 115.5.0)
FIXED - Error Console output when checking updates for add-ons not hosted on ATN (fixed in 115.4.2) - Bug 1861372
FIXED - After restart, unread/total message counts not shown for collapsed servers in the folder tree (fixed in 115.4.2) - Bug 1835548
NEW - Shift+Click on "Mark Folder Read" will also mark all sub-folders as read (fixed in 115.4.0) - Bug 500762
CHANGED - On Windows Betterbird now uses its own registry entries to better co-exist with a Thunderbird installation (fixed in 115.4.0)
FIXED - Bad X-MOZ-LASTACK attributes from iCloud CalDAV calendars not tolerated (fixed in 115.4.0) - Bug 1849849
FIXED - Couldn't open or save encrypted message attachment (fixed in 115.4.0) - Bug 1689326, Bug 1837247
NEW - Pref mail.download.open_attachments_readonly to disable that attachments are opened read-only (fixed in 115.3.2)
FIXED - Undo/Redo not working on stand-alone message window (fixed in 115.3.2) - Bug 1855643
FIXED - Fixed broken activation when clicking system tray icon on some Linux KDE installations (fixed in 115.3.2)
CHANGED - Improved activation when clicking system tray icon on Linux Mate (fixed in 115.3.2)
NEW - "Open Link in Browser" in compose window via context menu (fixed in 115.3.1) - Bug 695142
FIXED - SMTP: Limit messages per connection with new pref mail.smtpserver.default.max_messages_per_connection (fixed in 115.3.1) - Bug 1854567
RESTORED - Menu bar at the top of the main window (not underneath the new unified toolbar)
RESTORED - Total and unread/selected count removed from status bar (further improvements in 115.1.0, GitHub #183) - Bug 1831667
RESTORED - No more connecting lines in threaded view - Bug 1829470
RESTORED - Visual feedback during Quick Filter Search - Bug 1831500
RESTORED - Broken feed functionality, a lot of breakage in Thunderbird "Supernova": Feed updates, icons, etc.
MITIGATED - Problem with restoring message and folder tabs after restart on large profiles with many calendars - Bug 1841411
NEW - Vertically aligned tabs
NEW - Attachment list on top
NEW - Header pane resize
NEW - Minimize to system tray on Linux (KDE, Gnome and Xfce desktops), improved in 115.2.1
CHANGED - Improved portable launcher. Portable installation can now be moved without loss of functionality
CHANGED - New program icons for Mac (compliant with Apple rules)
CHANGED - Tab titles restore quicker (not a fix for tab restoral problem, see above)
CHANGED - More Troubleshooting Information now also displays identity and server keys
NEW - New preference gloda.list.immediate to show the search result as list immediately without having to click "Show results as list". Values: 0: Facet view, 1: Facet view and list view, 2: List view only. - Bug 580252
FIXED - Can't cancel sending message - Bug 1816540
FIXED better - Display/repair of some messages (incl. subject) with incorrect charset headers - Bug 1739609
FIXED - Can't drag contact on mailing list in other address book - Bug 1782397
FIXED - Invalid Atom feeds with missing title not accepted, for example German Heise c't feed
FIXED - Delivery status notification (DSN) request not working when "Send Later" is used - Bug 815638

Fixed in this version:
FIXED - Reply prefix ("Re:", "RE:") not detected for news messages - Bug 1845155
FIXED - Invalid Atom feeds with missing title not accepted, for example German Heise c't feed
Fixed in Betterbird, not fixed in Thunderbird 102.13.0 yet:
Fixes provided by the Betterbird project and integrated into Thunderbird to be released later
CHANGED - Improved the Sending progress dialogue: Now works during retry if previous dialogue was cancelled (fixed in 102.9.1) - Bug 1824624
FIXED - Hang when viewing HTML message with many links as plain text (possible denial of service attack vector) (fixed in 102.9.0) - Bug 1812386
FIXED - Subscribe/Unsubscribe buttons didn't check/uncheck the respective checkbox for the folder) (fixed in 102.7.1) - Bug 1813346
CHANGED - Selecting or unselecting a dictionary in the editor context menu doesn't close the menu (fixed in 102.0) - Bug 1761417
FIXED - Various problems when saving the compose window as file (fixed in 91.9.1) - Bug 1676825
FIXED - Status text sometimes displayed "null" (fixed in 91.9.0) - Bug 1766578
FIXED - .startup-incomplete not deleted from profile (fixed in 91.8.0) - Bug 1762043
FIXED - Inserting links with spaces gave random erroneous results (fixed in 91.0) - Bug 1563452
Various minor fixes that Thunderbird isn't shipping yet:
WITHDRAWN (as of 11th May 2023) - Problem with Google calendar descriptions - Bug 1792078
FIXED - Lost writing style on various occasions, for example after image insertion - Backout of bug 1730429
NEW - New preferences mail.biff.show_tray_icon_always to always show the system tray icon (in Betterbird effective for Windows and Linux) - Bug 1727874
FIXED - Cropping of subject lines with UTF-8 emoticons - Bug 1799093
FIXED - Send progress dialogue collapsed when sending encrypted message - Bug 1777788
CHANGED - Preference mail.smime.remind_encryption_possible set to true enabling the notification for possible S/MIME encryption - Bug 1808295
Other fixes: Bug 673703, Bug 1528297, Bug 1673446, Bug 1708320, Bug 1782415, Bug 1783517, Bug 1790619, Bug 1793313, Bug 1794117, Bug 1798597, Bug 1802685
Fixed in prior 102.x versions (Betterbird specific):
FIXED - Betterbird collected data for reporting without ever submitting it. Now data collection in the datareporting folder has been completely disabled. (fixed in 102.13.0)
FIXED - Update check didn't take policy DisableAppUpdate into account (fixed in 102.11.1) - GitHub #158
CHANGED - Linux: Using EGL 1.5 graphics now instead of GLX 1.4 fixing fuzzy fonts for some users (fixed in 102.9.1)
CHANGED - Linux: Now supporting sideloaded "application" add-ons stored in betterbird/extensions. To enable those add-ons, preference extensions.autoDisableScopes needs to be set to 3. (fixed in 102.9.1)
CHANGED - Multi-line support when add-on "Conversations" is used. Second line can be aligned with "Between" column. (fixed in 102.9.1)
CHANGED - Improved portable launcher. Can now be registered as default mail application on Windows. (fixed in 102.7.1)
CHANGED - Update check will now handle failure better (fixed in 102.6.1)
CHANGED - Followed Thunderbird by adding an "end-of-year donation appeal" (fixed in 102.6.0) - Bug 1801200
CHANGED - Reverted portable laucher to previous version so add-ons start more reliably without setting preference extensions.startupScanScopes to 5 or 7 (fixed in 102.5.1)
NEW - Re-enable additional languages (via Betterbird language packs) and language switching in the settings (fixed in 102.3.0)
NEW - Thunderbird language packs which are incompatible with Betterbird can no longer be installed (fixed in 102.3.0)
CHANGED - WM_CLASS on Linux is now "betterbird" (fixed in 102.3.0)
Features and fixes exclusive to Betterbird (new in the 102 series):
NEW - New preference gloda.list.immediate to show the search result as list immediately without having to click "Show results as list". Values: 0: Facet view, 1: Facet view and list view, 2: List view only (fixed in 102.13.0) - Bug 580252
NEW - Icon configurability for main window and system tray, compose window and calendar dialogues (add/edit calendar, add/edit/summary event) have their own icons. See here for details. (fixed in 102.11.0 and 102.11.1) - GitHub #151
FIXED - Stability fix for folder search and Quick Filter in encrypted messages (fixed in 102.11.1)
FIXED - Issue with complex search terms when inserting a term at the top (fixed in 102.11.0)
CHANGED - Pref browser.download.start_downloads_in_tmp_dir now set to true to avoid .partfiles on desktop when opening attachments from the compose window (fixed in 102.11.0) - Bug 1800328
NEW - Preference mail.startupMinimized to minimise the main windows after startup (aka "startup minimised") (fixed in 102.10.1)
NEW - Preference mail.multiselect_message_shows_current to disable summaries on multi-select and display the last selected message (fixed in 102.10.0) - Bug 364896
CHANGED - Newsgroup folders now display the last selected message when multiple messages are selected. This also applies when when preference mail.operate_on_msgs_in_collapsed_threads is set to false. (fixed in 102.10.0) - Bug 364896
NEW - Improved display of complex search terms in filter editor, search window and virtual folder/saved searches window (fixed in 102.10.0)
FIXED - Improved the Sending progress dialogue: Incompatibility with add-on MailMerge (fixed in 102.11.0) - GitHub #150
CHANGED - Improved the Sending progress dialogue, no longer showing 100% when sending is cancelled (fixed in 102.9.1)
CHANGED - Improved the Sending/Saving progress dialogue now showing 100% successful sending (fixed in 102.9.0)
NEW - Short date entry in Calendar with formats DD, DDMM, DDMMYY and DDMMYYYY, for example 0505 to mean the 5th of May of the current year (fixed in 102.8.0)
NEW - Regular expressions in search terms (folder search and filters) via "matches" and "doesn't match" conditions. Note that matching is case-insensitive and is implemented using the C++ std::regex library, which (almost completely) follows the regular expression syntax in the ECMA-262 Standard. (fixed in 102.7.1) - Bug 19442
NEW (Linux) - System tray support for Linux (fixed in 102.7.0) - Bug 18732
NEW - Insert menu for plain text compositions (fixed in 102.7.0) - Bug 230684
FIXED (Mac) - Dragging a message to the desktop didn't work (fixed in 102.7.0) - Bug 1455429
FIXED - Problems with invalid S/MIME signatures on servers which add empty lines (fixed in 102.6.1) - Bug 1805558
FIXED better - Multiple spellcheck dictionaries not reliably set in new composition and not restored when editing draft (fixed in 102.6.1) - Bug 1775376
NEW - Restored Unix Movemail support (Linux and macOS) (pending GitHub #104) (fixed in 102.5.1) - Bug 1728778 Bug 1802145
FIXED - Folder search and search/virtual folder not working when based on more than 500 folders, also causing MSF file loss (fixed in 102.5.0) - Bug 1554188
FIXED - Status display "Searching..." not cleared when messages loaded for search/virtual folder (fixed in 102.5.0) - Bug 561328
NEW - More menu options for selecting multi-line view: All folders, individual folders, switched off (fixed in 102.3.1)
FIXED - "Open in Conversation" not present on menus when collapsed thread was selected (fixed in 102.3.1) - Bug 668491
FIXED - Fix drag of image from MyPhoneExplorer (MPE) to compose window (fixed in 102.3.0) - Bug 1790291
NEW - Added non-breaking hyphen (U+2011) and em-dash (U+2014) to "Insert Character/Symbol" menu (en-dash was added previously) (fixed in 102.2.1)
FIXED - Compositions using "fixed width" text at a non-default size erroneously downgraded to plain text (fixed in 102.2.0 )- Bug 1786162
NEW - By popular demand, the multi-line view now also supports threading (fixed in 102.0)
CHANGED - Better styling for summary lines in "grouped by" multi-line views (fixed in 102.0)
NEW - Option to send message as HTML or plain text, depending on whether rich formatting is used (fixed in 102.0)
CHANGED - Unlike Thunderbird, Betterbird does not use a threaded view for new folders (preference mailnews.default_view_flags, 0 = normal, 1 = threaded) (fixed in 102.0)
FIXED - Some disturbing error console messages (fixed in 102.0) - Bug 1744944
FIXED - Multiple spellcheck dictionaries not reliably set in new composition and not restored when editing draft (fixed in 102.0) - Bug 1775376
Features and fixes exclusive to Betterbird (already in the 91 series):
NEW - Multi-line view. View > Layout > Multi-line View On All Folders or per folder in the column picker
NEW - Search terms now allow for complex expressions by adding a group feature - Bug 297852
NEW - Quick Filter untagged messages - Bug 683809
FIXED - Global search in encrypted messages (needs to be enabled in preferences) - Bug 188988
NEW - Regular folder search and Quick Filter in encrypted messages - Bug 1562737
CHANGED - Windows system tray ("systray") tooltip now showing list folders with new messages - Bug 1721063
NEW - Context menu in a news message now includes "Open Message For ID" (message pane only, not in message in tab)
FIXED - New posts in ignored threads caused "phantom" unread messages. "Mark > Thread As Read" will fix corruption - Bug 799040
NEW - Preferences mail.addressDisplayAuthor and mail.addressDisplayRecipients (1 = name, 2 = e-mail, 3 = both) for message list display - Bug 697031
NEW - Showing file URL for detached attachments, correct size and strike-through for deleted files - Bug 1740956, Bug 1741093
NEW - Folder context menu now offers to set folder as startup folder (instead of last visited)
NEW - Add Recent menu to allow attaching recently used files (Windows only) - Bug 1560737
FIXED - Issues when dragging Windows shell link (.lnk) or folder to Write (compose) window - Bug 276869, Bug 524874
CHANGED - Activity manager opens in tab
FIXED - Attachments saved via double-click didn't get listed in the list of saved files (Ctrl+J) - Bug 1744709
NEW - Ctrl+Shift+Space inserts NBSP now, also mostly fixes "automatic" unwanted NBSP replacement - Bug 532712
FIXED - Copying NBSP replaced NBSP with regular space (fixed in 91.8.0) - Bug 359303, Bug 1769534
FIXED - Repair Text Encoding didn't repair the subject - Bug 1739609
Not mentioned in the front page feature table:
NEW - Preference mail.openpgp.ignore_missing_mdc to allow display of attachments with missing MDC - Bug 1729221
NEW - Added preference mail.biff.show_for_server to optionally suppress biff notifications for server root folders.
FIXED - Subject change with "new subject (was: old subject)" didn't work - Bug 1680425
FIXED - Feed reader didn't tolerate any XML errors; now it will try fetching the feed as HTML as a fallback - Bug 1746749
FIXED - Moving multiple IMAP folders didn't work - Bug 589008
FIXED - Copying an embedded image didn't work under certain circumstances - Bug 1760365
FIXED - ... and a few more: Comma in detached file name (Bug 350825), Charset forward detection (Bug 1737245), Charset detection (Bug 1746695)

ased on Thunderbird 102.9.1, release notes.

Fixed in this version:

CHANGED - Improved the Sending progress dialogue, no longer showing 100% when sending is cancelled
CHANGED - Improved the Sending progress dialogue: Now works during retry if previous dialogue was cancelled - Bug 1824624
CHANGED - Linux: Using EGL 1.5 graphics now instead of GLX 1.4 fixing fuzzy fonts for some users
CHANGED - Linux: Now supporting sideloaded "application" add-ons stored in betterbird/extensions. To enable those add-ons, pref extensions.autoDisableScopes needs to be set to 3.
CHANGED - Multi-line support when add-on "Conversations" is used. Second line can be aligned with "Between" column.
Fixed in Betterbird, not fixed in Thunderbird 102.9.1 yet:

Fixes provided by the Betterbird project and integrated into Thunderbird to be released later

FIXED - Hang when viewing HTML message with many links as plain text (possible denial of service attack vector) (fixed in 102.9.0) - Bug 1812386
FIXED - Subscribe/Unsubscribe buttons didn't check/uncheck the respective checkbox for the folder) (fixed in 102.7.1) - Bug 1813346
CHANGED - Selecting or unselecting a dictionary in the editor context menu doesn't close the menu (fixed in 102.0) - Bug 1761417
FIXED - Various problems when saving the compose window as file (fixed in 91.9.1) - Bug 1676825
FIXED - Status text sometimes displayed "null" (fixed in 91.9.0) - Bug 1766578
FIXED - .startup-incomplete not deleted from profile (fixed in 91.8.0) - Bug 1762043
FIXED - Inserting links with spaces gave random erroneous results (fixed in 91.0) - Bug 1563452
Various minor fixes that Thunderbird ins't shipping yet:

FIXED - Improved IMAP error handling - Bug 1745130
FIXED - Lost writing style on various occasions, for example after image insertion - Backout of bug 1730429
NEW - New preferences mail.biff.show_tray_icon_always to always show the system tray icon (in Betterbird effective for Windows and Linux) - Bug 1727874
FIXED - Cropping of subject lines with UTF-8 emoticons - Bug 1799093
FIXED - Send progress dialogue collapsed when sending encrypted message - Bug 1777788
CHANGED - Pref mail.smime.remind_encryption_possible set to true enabling the notification for possible S/MIME encryption - Bug 1808295
Other fixes: Bug 1528297, Bug 1708320, Bug 1782415, Bug 1783517, Bug 1790619, Bug 1793313, Bug 1794117, Bug 1794180, Bug 1798597, Bug 1802685
Fixed in prior 102.x versions (Betterbird specific):

CHANGED - Improved portable launcher. Can now be registered as default mail application on Windows. (fixed in 102.7.1)
CHANGED - Update check will now handle failure better (fixed in 102.6.1)
CHANGED - Followed Thunderbird by adding an "end-of-year donation appeal" (fixed in 102.6.0) - Bug 1801200
CHANGED - Reverted portable laucher to previous version so add-ons start more reliably without setting pref extensions.startupScanScopes to 5 or 7 (fixed in 102.5.1)
NEW - Re-enable additional languages (via Betterbird language packs) and language switching in the settings (fixed in 102.3.0)
NEW - Thunderbird language packs which are incompatible with Betterbird can no longer be installed (fixed in 102.3.0)
CHANGED - WM_CLASS on Linux is now "betterbird" (fixed in 102.3.0)
Features and fixes exclusive to Betterbird (new in the 102 series):

CHANGED - Improved the Sending/Saving progress dialogue now showing 100% successful sending (fixed in 102.9.0)
NEW - Short date entry in Calendar with formats DD, DDMM, DDMMYY and DDMMYYYY, for example 0505 to mean the 5th of May of the current year (fixed in 102.8.0)
NEW - Regular expressions in search terms (folder search and filters) (fixed in 102.7.1) - Bug 19442
NEW (Linux) - System tray support for Linux (fixed in 102.7.0) - Bug 18732
NEW - Insert menu for plain text compositions (fixed in 102.7.0) - Bug 230684
FIXED (Mac) - Dragging a message to the desktop didn't work (fixed in 102.7.0) - Bug 1455429
FIXED - Problems with invalid S/MIME signatures on servers which add empty lines (fixed in 102.6.1) - Bug 1805558
FIXED better - Multiple spellcheck dictionaries not reliably set in new composition and not restored when editing draft (fixed in 102.6.1) - Bug 1775376
NEW - Restored Unix Movemail support (Linux and macOS) (pending GitHub #104) (fixed in 102.5.1) - Bug 1728778 Bug 1802145
FIXED - Folder search and search/virtual folder not working when based on more than 500 folders, also causing MSF file loss (fixed in 102.5.0) - Bug 1554188
FIXED - Status display "Searching..." not cleared when messages loaded for search/virtual folder (fixed in 102.5.0) - Bug 561328
NEW - More menu options for selecting multi-line view: All folders, individual folders, switched off (fixed in 102.3.1)
FIXED - "Open in Conversation" not present on menus when collapsed thread was selected (fixed in 102.3.1) - Bug 668491
FIXED - Fix drag of image from MyPhoneExplorer (MPE) to compose window (fixed in 102.3.0) - Bug 1790291
NEW - Added non-breaking hyphen (U+2011) and em-dash (U+2014) to "Insert Character/Symbol" menu (en-dash was added previously) (fixed in 102.2.1)
FIXED - Compositions using "fixed width" text at a non-default size erroneously downgraded to plain text (fixed in 102.2.0 )- Bug 1786162
NEW - By popular demand, the multi-line view now also supports threading (fixed in 102.0)
CHANGED - Better styling for summary lines in "grouped by" multi-line views (fixed in 102.0)
NEW - Option to send message as HTML or plain text, depending on whether rich formatting is used (fixed in 102.0)
CHANGED - Unlike Thunderbird, Betterbird does not use a threaded view for new folders (preference mailnews.default_view_flags, 0 = normal, 1 = threaded) (fixed in 102.0)
FIXED - Some disturbing error console messages (fixed in 102.0) - Bug 1744944
FIXED - Multiple spellcheck dictionaries not reliably set in new composition and not restored when editing draft (fixed in 102.0) - Bug 1775376
Features and fixes exclusive to Betterbird (already in the 91 series):

NEW - Multi-line view. View > Layout > Multi-line View On All Folders or per folder in the column picker
NEW - Search terms now allow for complex expressions by adding a group feature - Bug 297852
NEW - Quick Filter untagged messages - Bug 683809
FIXED - Global search in encrypted messages (needs to be enabled in preferences) - Bug 188988
NEW - Regular folder search and Quick Filter in encrypted messages - Bug 1562737
CHANGED - Windows system tray ("systray") tooltip now showing list folders with new messages - Bug 1721063
NEW - Context menu in a news message now includes "Open Message For ID" (message pane only, not in message in tab)
FIXED - New posts in ignored threads caused "phantom" unread messages. "Mark > Thread As Read" will fix corruption - Bug 799040
NEW - Preferences mail.addressDisplayAuthor and mail.addressDisplayRecipients (1 = name, 2 = e-mail, 3 = both) for message list display - Bug 697031
NEW - Showing file URL for detached attachments, correct size and strike-through for deleted files - Bug 1740956, Bug 1741093
NEW - Folder context menu now offers to set folder as startup folder (instead of last visited)
NEW - Add Recent menu to allow attaching recently used files (Windows only) - Bug 1560737
FIXED - Issues when dragging Windows shell link (.lnk) or folder to Write (compose) window - Bug 276869, Bug 524874
CHANGED - Activity manager opens in tab
FIXED - Attachments saved via double-click didn't get listed in the list of saved files (Ctrl+J) - Bug 1744709
NEW - Ctrl+Shift+Space inserts NBSP now, also mostly fixes "automatic" unwanted NBSP replacement - Bug 532712
FIXED - Copying NBSP replaced NBSP with regular space (fixed in 91.8.0) - Bug 359303, Bug 1769534
FIXED - Repair Text Encoding didn't repair the subject - Bug 1739609

Not mentioned in the front page feature table:
NEW - Preference mail.openpgp.ignore_missing_mdc to allow display of attachments with missing MDC - Bug 1729221
NEW - Added preference mail.biff.show_for_server to optionally suppress biff notifications for server root folders.
FIXED - Subject change with "new subject (was: old subject)" didn't work - Bug 1680425
FIXED - Feed reader didn't tolerate any XML errors; now it will try fetching the feed as HTML as a fallback - Bug 1746749
FIXED - Moving multiple IMAP folders didn't work - Bug 589008
FIXED - Copying an embedded image didn't work under certain circumstances - Bug 1760365
FIXED - ... and a few more: Comma in detached file name (Bug 350825), Charset forward detection (Bug 1737245), Charset detection (Bug 1746695)