[laktak]

So is Thunderbird still using XUL or not?

[rndgermandude]

Yes and so is Firefox. So far they only removed XBL not XUL, and while they are slowly migrating things to HTML most of the UI is still XUL.

[pas]

If yes it's only a matter of time until it transitions away from it too.

[baroffoos]

Likely only because it doesn't see enough attention to be migrated.