XUL is only used for the external chrome interface of the Firefox desktop browser, let's be clear not to commingle the technology used to build the UI of Firefox with the standards-based technologies that power web apps/pages ;)
https://developer.mozilla.org/en-US/docs/XULRunner
The XML-based XUL isn't terrible, either, considering the respectable set of widgets and the ability to control all the styles with CSS. The nasty part, in my experience, was trying to get through the messy FFI that was XPCOM.