What about the possibility to select the domain "depth" without using multiple menu entries, but selecting them with the mouse? For example, if you are on http://forums.informaction.com, clicking on the "informaction" you'll allow informaction.com, clicking on "forums" you'll allow forums.informaction.com. Here's a screenshot of a GIMP made example:

This screenshot should suggest you I have no idea how to implement it and if this is complex to do with XUL...