T-bird 6 has been released. Update from with the program or download it.
The latest version of Thunderbird has the following changes:
Thunderbird is based on the new Mozilla Gecko 6 engine
Several theme improvements for Windows 7
Support for Windows 7 Jump lists
Several fixes when importing email from Microsoft Outlook
Default mail client check now works with newer Linux distributions
Various other user interface fixes and improvements
Numerous platform fixes that improve speed, performance, stability and security