I just came across this very informative article on creating web applets with XUL. It provides an introduction to XUL and Mozilla as a development platform, in the form of a tutorial for creating an online helpdesk system.
Now I gotta tell you, I spent a good many days playing with XUL over the summer, and it can be a real bitch to work with. However, it's a very powerful, cross-platform bitch. It's well worth learning, whether you want to create online applets, or offline applications.
Check out the full tutorial.