One of the new features that is still on the Thunderbird 2.0 Road Map is ‘Tabbed Messages’. The way this feature was suppose to work was allow you to open/view multiple messages in the preview pane switching between tabs. Presently, each message when opened, opens in its own window. Take a look at this screen-shot from bugzilla bug 297379 to get an idea of how this would work. There is another screen-shot showing an interesting idea on how to use tabs on the compose screen. Unlike the first screen-shot, the second one is from Sylpheed-Claws, which is an open-source mail client based on GTK+. It UI is a bit more advanced than Thunderbird’s and does provide some inspiration for integrating tabs into Thunderbird.
Now, looking back through Bug 297379 one notices it was started back in June 2005 by Myk Melez. Digging through the comments in the bug report we learn in comment #52 Myk was had been assigned away from the Thunderbird 2.0 project. He had hoped someone would pick-up where he had left off on this idea/bug, but it appears no one has.
For those wondering looks like Thunderbird 2.0 is going to be coming out after Firefox 2.0. The release schedule on the Roadmap shows late November or early December. Not surprising since Thunderbird is still officially (as per milestones) only in the Alpha stage. Although they did release Pre-Beta 1 builds over the past two weekends those are only nightlies and not milestone releases.