Actually, Thunderbird was never on XULRunner; it always relied on linking all of its code into libxul. For some strange reason, many of the C++ string APIs are different depending on when you linked into or outside of libxul, which meant it was a lot of chore to maintain a build that worked on XULRunner, and the project was never completed before Mozilla killed XULRunner in favor of embedding stories that never lasted more than a year or two.
