XUL is not html, and you'll have to hit the docs every time you need something. Also, debugging XUL is a pain. XUL is pretty ancient too, so that probably doesn't help either.

Sticking with plain jane html is definitely the better approach, however I doubt the XUL implementers would have ever known that back in the year 2000. There's still a bunch of tools for XUL, but fairly rusty: http://xul.sourceforge.net/