The latest version of Thunderbird Beta has the following changes:

Add-ons installed by third party programs are now disabled by default
Added a one-time add-on selection dialog to manage previously installed add-ons
Improved accessibilty of the attachment lists
Several user interface fixes and improvements
and numerous other platform fixes

Known Issues
This list covers some of the known problems with Thunderbird Beta. Please read this before reporting any new bugs.

All Systems
If you are unable to view content for your RSS feeds in the Wide View Layout, you may need to disable the Lightning Calendar add-on if you have it installed or switch to Classic View and restart Thunderbird. (Bug 531397)