QUOTE
Test builds of Mozilla Thunderbird with an updated patch for tabbed message browsing are available. Developed by Myk Melez, this feature allows users to right-click a message header and select "Open Message in New Tab" from the context menu. They can also update advanced preferences to make the Enter key open messages in tabs. Trunk and 1.5 branch builds with the patch applied are available at Myk's blog.
I wouldn't mind testing this out. There are some screen shots available, too:
http://www.flickr.com/photos/69903184@N00/70869729/
https://bugzilla.mozilla.org/attachment.cgi?id=186166