I think that Safari is the fastest browser on Mac and above all it is more integrated with OSX core. Probably because it is the only one on 64 bit, or maybe because is OSX native, I don't know, but the result is that managing many tabs on Firefox causes my computer going out of memory (is frequent to have more than 800MB of RAM used by Firefox).
On the contrary Safari is really faster, but it has 2 major problem.
1. I don't like mad personalizations, but tools like the dig toolbar or firebug are really useful! I really miss extra toolbar on Safari.
2. Managing login & password. Is not clear where is the line between usability and privacy, but Firefox manages cookie and autocomplete really better than Safari, where I have to re populate every time!