The newest Thunderbird 3.0a1pre nightly (062707) now has tabs support. It doesn’t work very well, but it is important to keep in mind TBird 3.0 is still in very early development (not even an official Alpha release yet). A couple minor quirks, the close tab buttons are there but don’t do anything (need to right click on the tab and select “Close Tab’). Also double-clicking a message will cause it to open in a new window. You will need to right-click on the message and select ‘Open Message In New Tab’ option.
TBird 3 is also very unstable as it only took a couple minutes of me playing around with it to get it to crash. Also looks like there are issues with Talk Back (which I thought was replaced by Airbag/Break Pad) as I was getting the familiar memory writing error when I exited the application. I’ll be sticking with my 18.104.22.168pre build for a while still but wanted to try this out.
Edit: Added a couple screenshots. Also noticed all the tabs will sometimes reset to whatever folder you are currently navigating in.
News Source: Mozilla Links