Safari 4 on the Mac, of course. Firefox 3.5 on non-Macs.
I've got Firefox on the Macs, too, but only fire it up to compare a website if I think there might be a Safari bug or far more likely, the website mangled their HTML in a way that isn't Safari-friendly (and their webmaster earns a really dirty look from me).
I've got Firefox on the Macs, too, but only fire it up to compare a website if I think there might be a Safari bug or far more likely, the website mangled their HTML in a way that isn't Safari-friendly (and their webmaster earns a really dirty look from me).