Release notes for 6.0:
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
Release notes for 5.0:
More responsive and faster to start up and use
Thunderbird is based on the new Mozilla Gecko 5 engine
New Add-ons Manager
Revised account creation wizard to improve email setup
New Troubleshooting Information page
Tabs can now be reordered and dragged to different windows
Attachment sizes now displayed along with attachments
Plugins can now be loaded in RSS feeds by default
There are several theme fixes for Windows Vista and Windows 7
Support for Mac 32/64 bit Universal builds (Thunderbird no longer supports PowerPC on Mac)
Over 390 platform fixes that improve speed, performance, stability and security
