Mac OS X Lion users will also be glad to know that the full screen mode is now fully supported, and all users can also now expect better mouse performance in web-based games and other applications thanks to Mozilla's implementation of the Pointer Lock API.
Thunderbird: Stability and Community Innovation | Mitchell's Blog
Much of Mozilla’s leadership — including that of the Thunderbird team — has come to the conclusion that on-going stability is the most important thing, and that continued innovation in Thunderbird is not a priority for Mozilla’s product efforts.
Too bad. Personally, I think there's plenty of room for innovation in desktop mail clients, even with the buzz around mobile clients.