XUL was a horrible idea, certainly at the time. Firefox's UI felt sluggish due to it, certainly since I was used to Opera's blazing fast and responsive UI, which I stuck to for exactly that reason. I only had Firefox installed for compatibility reasons. People seem to forget that while IE was a mess, Firefox was only "better", not "amazingly fantastically better".

With computers having become faster, together with development thrown at it, that's not really an issue anymore, but developers have also realized these days that XML is far from human readable, and for the majority of applications it has been used for, simply a bad idea.