I thought FireFox was plenty fast enough on my C2D iMac, but check out these builds. It can't use the Firefox icon or name since it's technically modified (It uses the blue globe and identifies itself as "BonEcho"), but damn is it fast! So...optimized build above (which gives you Aqua widgets, by the way), plus: UNO to unify your interface, plus the Pinstripe Firefox theme (smoothest looking theme on OS X), plus optionally the Stop-Reload extension, plus the ever-essential Tab Mix Plus extension, and you've got the fastest, smoothest, best-looking, most extendable browser on OS X. Seriously, give the optimized BonEcho with Aqua widgets a try. EDIT: Also have to throw in another extension I use but temporarily forgot about, Fission. It turns your address bar into a status bar a'la Safari.