I suspect it's because the XUL used in Firefox and later is not "pure". I seem to recall that one of the big "optimizations" that FF did compared to the mozill

I suspect it's because the XUL used in Firefox and later is not "pure". I seem to recall that one of the big "optimizations" that FF did compared to the mozilla suite (these days known as Seamonkey) was to translate a bunch of UI elements into native equivalents rather than drawing it all from XUL. Thus replacing that with HTML may well be closer to reverting to the pre-FF XUL way.