Yes and no. Mozilla's a bit screwed on this front, because they use XUL to render their interface - and, critically, the browser can render XUL pages. I don't have FF installed on this machine, but you should still be able to check it out at http://www.faser.net/mab/remote.cfm to see a demo of the feature.
It's a pretty cool feature, but it means that on Firefox, attackers should be able to emulate basically any chrome they want to.
(... which were easy to spot for me, because I'm using a completely different system)