This has worked on Firefox since the beginning of time. One could use the DOM inspector and venkman JS debugger, then Firebug chrome inspector and more recently the Firefox developer tools to inspect Firefox's XUL.
Ha, weird, I never felt the same direct access to the metalevel as in Chrome. Just the other day I spend an afternoon trying to access Firefox restore session url list through the developper tools without any luck, the XUL ~DOM element was an opaque type I couldn't inspect.
https://developer.mozilla.org/en-US/docs/Tools/Tools_Toolbox...