http://www.getthunderbird.com/
Thunderbird 3.1, based on the Gecko 1.9.2 platform, was built by a
community of passionate contributors, experienced developers, security
experts, localization and support communities, and thousands of active
testers.
Some of the new features:
New Quick Filter toolbar
New Migration Assistant
Saved Files Manager
Several fixes to improve upgrading from Thunderbird 2
Several design improvements and corrections to the interface
Stability, memory, and password handling improvements
As always Thunderbird 3.1 is available as a free download:
http://www.getthunderbird.com/
Thunderbird 3.1 is also available in 46 languages – get your local version:
http://www.mozillamessaging.com/en-US/thunderbird/all.html
We encourage developers to read the Thunderbird 3.1 for Developers
article on the Mozilla Developer Center.
https://developer.mozilla.org/en/Thunderbird_3.1_for_developers