I haven't used a Mozilla product in a while, but was prompted to do so today with the news of 0.7.1's release for Panther compatibility. It doesn't launch quite as fast as Safari, but once it's going navigation seems to be just as fast. Page rendering, however, is a tad slower.
Even though Firebird is a lightweight version of Mozilla, it still sports more features than Safari. If not for the speed issue, it'd be a toss-up as to which would become my primary browser.