I am a Vista user, I’m not very proud of it but can’t afford a Mac at the moment, and use Firefox as my favorite browser. Works like it should, but I have one problem with it. The menu buttons are in a real ugly blue, see it for yourself:
I not really in to using themes for your browser, because most of the time they just slow down your system. But for one time I wanted to make an exception.
If you like the blueish Vista like buttons you can try the Vista on XP addon.