How to use Thunderbird with GUMail: New Features in Thunderbird 2.0:
Session History Navigation

This feature allows a user to navigate back and forth through their message history. This is particularly useful, if you happen to navigate away from your current messgae, and need an easy, quick way to return to it.

Other new and improved features can be read about at http://www.mozilla.com/en-US/thunderbird/2.0.0.0/releasenotes/.