First of all, you might want to get the optimized build for your CPU type: G4, G5 or Intel. So come and download them here. This is for Firefox 2.0, and follow up the site when there is an update.
Follow the link above, you could also found some very nice theme for Firefox to integrate into Mac OS X desktop. I strongly suggest Pinstripe theme and Aronnax’s Firefox Theme. The former theme feature modified Firefox toolbar icons that resemble OS X style. And the later one will transform your Firefox into Safari style.
Here is the official screenshot of Pinstripe theme.
Although I am a die-hard Opera fan, sometimes using Firefox is OK and sometimes I have to because some websites are still not compatible with Opera, such as Google Doc.