To my knowledge, it is completely posible to write helper apps in JavaScript to function with XUL. At least as extension modules for apps already using the whole thing. For example I think that ChatZilla extension for mozilla is written that way, in pure JavaScript. But if XUL can be used as a core part of some system's dialog/windows rendering I think it is completely possible to create relatively simple facility to have complete standalone apps in JavaScript. It's a long time since I've played with XUL, but if you have FireFox, you can start to play with it right now, and create for example simple dialogs & windows with radio buttons etc using that language and CSS styles.
PS: Yes it was an accindent, I wanted to send the mail to the list, sorry ;).