This has to do with running JavaScript, which is pretty much on every single site you'll visit these days. Well, I Google'd for a site without JavaScript and found this.

It looks like running Firefox 3 Beta 5 is faster than Sarari. However, to get this to be the case, you have to be running it without extensions. Personally, I run Firefox BECAUSE OF its extensions. If you want the FASTEST Mac OS X browsing experience, then you'd have to run WebKit's latest.

The following graph is courtesy of ArsTechnica.

Revised JavaScript benchmarking results for Firefox 3b5.

Firefox 3b5 vs Safari 3.1 Round 2: Disable Extensions