>>14
Gecko had few stability issues, and had and still has one of the best standards implementation, going as far as the most useful parts of CSS3 already done. Gecko is pretty damn fast, and still is. It was surely an improvement over MSIE, which was previously faster. It's Firefox' UI what's slow. Compare K-Meleon.
XUL is actually a good idea for distributed applications with lightweight, smart clients; your only other choice is the web and it was not designed for it; web applications suck sure there's AJAX, but it's made of dirty and hack, there are few actually useful frameworks where you wouldn't have to do everything by hand, and it's a PAIN IN THE ASS to do, which means it's IMPRODUCTIVE.
XUL's problem is it was designed like business software, and ended up being bloated and enterprise'd a bit, plus there's the whole web 2.0 services shiz behind it. On top of that, its documentation must be the worst ever; it's as bad as having a fetus shoven up your ass twice.