Mozilla released Firefox 4 just recently and hit over 6 million downloads so far. It promised speed and security, so far the speed part is pretty evident. It took just one bounce on the dock and the window appeared. Website loading appears to be fast, it matches Chrome's rendering power.
The UI is more streamlined now almost like Chrome's minimalist design, very useful on Macbook Air's smaller screen estate. It is also capable of running full screen, with a smoother transition than Chrome.
In my short experience with Firefox, I think it is a good alternative to Chrome, especially on the Mac OS platform. There are a couple of sites that don't render well on Chrome, so hopefully they run on Firefox.
Hope I can truly snowball Safari which frankly doesn't appeal to me, never has. Apple gives you a nice big widescreen but insists it is silly to view the internet in full panorama.