A user of one of the "Thunderbird 1.5 with tabs" builds I made a couple years ago recently asked me if it would be possible to update the patch for Thunderbird 2.0. After some wrangling, I got the patch to apply and behave itself in my testing, and I've spun some tab-enabled builds of 126.96.36.199 for Linux, Mac OS X, and Windows.
Note that the Thunderbird developers are implementing a different version of tabs for Thunderbird 3 over in bug 218999. So I won't be porting this patch to the trunk. But if you can't wait for that release to get your tabs, these builds may be useful to you.
To open a message in a tab, context-click it and select "Open message in new tab" from the context menu. To make this the default behavior when opening messages, set Preferences > Advanced > General > "Open new messages in" to "A new message tab".
p.s. congrats to Thunderbird on the new organization! I'm looking forward to great things from it.