My understanding is that part of the goal with this UX rewrite is to move from XUL to HTML/CSS as XUL is increasingly unmaintained as Firefox has made the move for most (maybe all by now) of it's UI years ago
As I understand it, Thunderbird is essentially Firefox turned into a mail client. So whatever FF is using. There's probably still some XUL left here and there, but only because they haven't gotten around to replacing it yet.
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.
EDIT: On 2nd thought I'm thinking this is XUL, Mozilla's sort of electron equivalent for an HTML/CSS UI?