Thunderbird 3.1.3: http://getthunderbird.com/
Thunderbird 3.0.7:
http://www.mozillamessaging.com/en-US/thunderbird/all-older.html
We strongly recommend that all Thunderbird users upgrade to these latest
releases. If you already have Thunderbird, you will receive an automated
update notification within 24 to 48 hours. You can also manually fetch
this update by selecting “Check for Updates…” from the Help menu.
For a list of changes and more information, please review the
Thunderbird release notes:
Thunderbird 3.1.3 release notes:
http://www.mozillamessaging.com/en-US/thunderbird/3.1.3/releasenotes/
Thunderbird 3.0.7 release notes:
http://www.mozillamessaging.com/en-US/thunderbird/3.0.7/releasenotes/