How to use Thunderbird with GUMail: New Features in Thunderbird 2.0:
Mail Message Notification Window

 

In previous versions, users were only given notification that they had a new message. However, they were not aware of the contents of the message. With version 2.0, mail notification alerts now display the subject of the message, sender, and a preview of the body of the message.

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