XUL would have had worse issues because it could make arbitrary XPCOM calls to all sorts of native components and nearly the full gamut of native component issues written mostly in C/C++.
XUL was in many ways always a ticking time bomb.
The current frontend still has the same XPCOM privilege access from JS, so as emiliocobos said, XUL vs. HTML does not change the security boundary. It's only a different markup language.
It still surprises me parts of Firefox still use XUL.