I work on an extension for Chrome, Opera, Safari, and Firefox that's pretty widely used. Firefox (Jetpack) is so different from the rest that I basically hate working on it.

I get that some people may love XUL and whatever, but having never used it, I can't really comment. All I do know is that Jetpack (which Firefox has been pushing) is a sorry excuse for a way to build extensions after you've worked with Chrome. I really hope WebExtensions makes my life easier soon.