SeaMonkey 2.6.1 contains the following major changes relative to SeaMonkey 2.5:

SeaMonkey-specific changes
Added support for the HTML5 "context menu" feature (contextmenu attribute)
Added Print Preview support to Composer
Added update channel indication to About SeaMonkey page (about:)
Mozilla platform changes
Added Type Inference, significantly improving javascript performance
Added support for querying Do Not Track status via javascript
Added support for font-stretch
Improved support for text-overflow
Improved standards support for HTML5, MathML, and CSS
Fixed several stability issues

* Sync: add option to cleanup old generations and
old entries for deleted files, older that a year (configurable).
* Fix File Monitoring not working for root folder of a disk.
* Browse dialog: New Folder makes it current.
* S3: make host-based adressing to be default option.
* S3: allow region-specific hosts in URL-based addressing.
* Azure: added check for illegal '.' char at end of filename.
* SkyDrive: Added password length check (6-16 required).
* SkyDrive: file types SD does not like now have ._gs_ appended to them.
* Make time that is out of range warning more specific.
* Server: introduce dynamic Forwarders that are user's computers.
* Mediator: speed up talkback, so that GS server is registered faster.
* Server: new better generic self-signed certificate for SSL.
* Server: report its version to GS client.
* Server: do not use GSTP verbs directly in HTTP, so that it can go thru proxies.
* Server: fix automatic reconnection of file system after lost connection.
* Server: sessions now timeout after 3 minutes of inactivity.