Safari / Webkit is clearly the fastest and most sublime browsing experience you can have on a Mac (or a PC.) Opera died a long time ago, unfortunately, as they continued to innovate the browsing experience forward; but they did it at the expense of a clunky and bloated user experience.
The only place where Firefox 3.x is superior to Webkit is it's established and dedicated developer community for plug-ins and the more accessible plug-in architecture w/r/t Webkit.
Webkit's "Squirrelfish" (cringe at the name if you have to) ECMAScript rendering engine is the fastest and most robust of any browser and the fastidious adherence to standards (scoring 100 on the Acid 3 browser test (www.acid3.org) are making FF dig severely deep in the code tree to prune out abject code. In contrast, Webkit does not allow *any* code to be checked in that is _slower_ OR _slows down_ existing code through the nightly regression test policy.
I love the fact that I can download a nightly release of Webkit KNOWING it's going to be faster than yesterday's version, more stable, and more secure. I can choose to upgrade whenever I want to -- on Apple's release schedule or my own and absolutely know that every night the browser gets *better* (even if al that means is a slight refactoring check-in, etc.)
Now I used to use IE7 / 8 beta on Vista x64 because it _felt_ fast being pre-loaded during intital boot -- but then I began using Webkit (Safari) and suddenly my eyes were opened. When I switched primarily to Mac I never looked back.
I always had to use FF + IE on Windows to test code, etc. On my Mac, not so much. Just Webkit. Obliterated my virtual machines and my Bootcamp setup months ago and never have regretted it. (OK, I can't play Civ IV BTS... which is kinda a bummer, but I'm rocking Spore now anyways).
What Firefox needs to do to catch up is to get their runtime size down, start-up time faster, and *handle tabs as separate processes!* Oh yeah, and score a perfect 100 on Acid 3. That is competition, folks.
Cheerio,
-- f9a