Log Message:
Update to Thunderbird 2.0.0.0.

What's New in Thunderbird 2

* Message Tags: Create your own tags for organizing email. Messages can be
  assigned any number of tags. Tags can be combined with saved searches and
  mail views to make it easier to organize email.
* Visual Theme: Thunderbird 2's theme and user interface have been updated to
  improve usability and maximize screen real estate.
* Session History Navigation: Back and Forward buttons allow navigation through
  message history.
* Advanced Folder Views: Customize the folder pane to show favorite, unread or
  recent folders.
* Easy Access to Popular Web Mail Services: Gmail and .Mac users can access
  their accounts in Thunderbird by simply providing their user names and
  passwords.
* Improved Support For Extensions: Extensions can now add custom columns to the
  message list pane in addition to storing custom message data in the mail
  database.
* Improved New Mail Notification Alerts: New mail alerts include information
  such as the subject, sender and message preview text.
* Folder Summary Popups: Mouse over a folder with new messages to see a summary
  of the new messages in that folder.
* Saved Search Folder Performance: Search results for saved search folders are
  now cached, improving folder loading performance.
* Find As You Type: Finds and highlights message text as you type.
* Improved Filing Tools: Recent folder menu items for moving and copying
  messages to recently used folders. Move / Copy again functionality.
* Updates to the Extension System: The extension system has been updated to
  provide enhanced security and to allow for easier localization of extensions.

The Rumbling Edge has a more detailed list of notable bug fixes:
http://weblogs.mozillazine.org/rumblingedge/archives/2007/03/tb_2.html