The latest version of Thunderbird Beta has the following changes:

New Addons Manager and extension management API (user interface will be changed before final release)
Tabs can now be reordered and dragged to different windows
Revised account creation wizard, offering improved set-up
Attachment sizes now displayed along with attachments
New troubleshooting information page to aid supporting and diagnosing problems in Thunderbird
Plugins can now be loaded in RSS feeds by default
Various other user interface fixes and improvements
Support for Mac 32/64 bit Universal builds (Thunderbird Beta will no longer support PowerPC on Mac)
and numerous other bug fixes