Anticipating requests, I've spun some tab-enabled builds of the latest version of Mozilla's mail/news desktop app for Linux, Mac, and Windows.
Note that the Mac version is no longer compatible with Mac OS X 10.2, as my Mac build machine (a.k.a. my laptop) has been upgraded to Leopard and XCode 3.0 and no longer has the 10.2.8 SDK on it. It should work on 10.3.9+, 10.4, and 10.5, however.
p.s. Congrats to the Thunderbird team on the release of 188.8.131.52! Getting a security and stability release out the door is no mean feat, requiring careful coordination between several groups of talented folk with a high standard for software quality that they consistently achieve. Bravo!