Of course it does. Microsoft is very good with backwards compatibility, especially from NT onward, and that's assuming Mozilla wasn't interested in supporting their most commonly used platform (I'm pretty sure Windows is). This is just talking about the port to Metro, which has seen poor reception.
Of course I doubt Firefox would have been a "true" Metro app... I don't think Chrome was... as part of MS' attempt to be anti-competitive with web browsers in Windows 8, they allow the default web browser to inject itself into Metro, but still run outside of the sandbox (otherwise, they would have to use the IE rendering engine! At least AFAIK). But you still want the UI to look Metro. Anyway, if the browser is not the default, it can still run but only on the desktop in its traditional UI. This restriction also applies to IE.