Release notes: http://www.mozilla.org/en-US/thunderbird/15.0/releasenotes/
- Multi-Channel Chat: You now can enjoy real time conversation with your contacts, right from your favorite messaging application.
- Do Not Track: This option has been implemented as an addition to Search the Web.
- Ubuntu One is now supported in Filelink – the option to upload large attachments to online storage services.
- New User Interface: Thunderbird is replicating the new look and feel of Mozilla Firefox in an effort to provide a similar user experience across all Mozilla software desktop or mobile and all platforms.
If you do not receive an update notice when using the application, select “Check for Updates” from the Help menu.
Or download: http://www.mozilla.org/en-US/thunderbird/all.html