Thunderbird 3.0.11 is the last security and stability update for 
Thunderbird 3.0.x. Thunderbird 3.0.x users will be prompted and 
encouraged to start using Thunderbird 3.1 starting early next year.
Thunderbird 3.1.7: http://getthunderbird.com/
Thunderbird 3.0.11: 
http://www.mozillamessaging.com/en-US/thunderbird/all-older.html
We strongly recommend that all Thunderbird users upgrade to these 
releases. If you already have Thunderbird 3.1 or 3.0, 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 3.1.7 release notes: 
http://www.mozillamessaging.com/en-US/thunderbird/3.1.7/releasenotes/
Thunderbird 3.0.11 release notes: 
http://www.mozillamessaging.com/en-US/thunderbird/3.0.11/releasenotes/