Every actual test and benchmark comparing the major Mac browsers that I've ever seen show that Safari is not only faster than FireFox and all the other Mozilla browsers, but significantly faster. Opera too.
Sure, one browser might display MySpace faster while another displays eBay faster, but overall, Safari is the fastest hands down, on all Macs. Don't fool yourself into thinking a certain browser is fastest based on a single page.
I know personally I have used FireFox, Camino, Safari, OmniWeb, Opera, and even iCab extensively, and each one has pages where it seems like "wow this browser is so faster" ... but in reality it isn't on a majority of pages.