The latest release of Thunderbird has a different look and new features.
Tabs
If you like Firefox’s tabbed browsing, you’re going to love tabbed email. Tabbed email lets you load emails in separate tabs so you can quickly jump between them. Perhaps you’re responding to an email and need to refer back to an earlier email. Tabbed email lets you keep multiple emails open for easy reference.
Search
The new search interface in Thunderbird 3 contains filtering and timeline tools to pinpoint the exact email you’re looking for. Thunderbird 3 also indexes all of your emails to help you search even faster.
Message Archive
If you think you’re going to need an email in the future but want it out of your inbox without deleting it, archive it! Archiving helps you manage your inbox and put your email into a new archive folder system.
http://www.mozillamessaging.com/en-US/thunderbird/3.0/releasenotes/
Update from within Thunderbird or download
