To get rid of XUL of course, which is a big reason the memo says it's being canceled. (Might be the biggest reason, since it's mentioned first, but the memo wanders around and is pretty vague on exactly what the problems are)
XUL sits on top of a toolkit like Qt or GTK. There's Qt ports of Firefox FWIW (which are still XUL based), or at least there were when Nokia was still a thing.
