New in Thunderbird 3.0 Beta 1 is a tab interface for mail, IMAP improvements, improved message reader view, new add-ons manager, improved address book interface, improved import of mail from other mail clients, integration with Windows Vista search, and integration with the address book found in Mac OS X. The IMAP improvements should yield faster message viewing.
The Thunderbird 3.0 Beta 1 announcement can be read at Mozilla Messaging along with the release notes. David Ascher, the leader of Mozilla Messaging, has also written a detailed blog post that describes many of the changes found in this release along with screenshots from the Mac OS X build.
It looks we will have Thunderbird 3.0 to look forward to in 2009.