Firefox doesn't run on a Mac. I use Firefox at work on my Dell, and Camino at home on my beloved iMac.
I'm delighted with both.
The Mozilla Foundation produces a browser especially for the Mac platform, called Camino. (Of course, you could just run firefox as a X application, since OS/X is really BSD Unix under the hood, but really, you should give Camino a try).
Firefox does too run on Mac!