Mozilla Messaging has released the first Beta of Thunderbird 3.1, code named Lanikai. This release includes:

From the release notes:

• Several fixes to improve upgrading from Thunderbird 2.
• Several fixes for auto complete, tabs, and activity manager.
• Several design improvements and corrections to the interface.
• Stability and memory improvements.

You can try Thunderbird 3.1 from Mozilla Messaging available for Windows, Mac OS X, and Linux in 25 languages.

Source: Mozilla Links