I tried. I really tried to use Firefox as my default browser. I was kind of enjoying it, but kept finding myself tripping over stuff like a UI that doesn't respond the way a native MacOSX app should, and a browser that was rather prone to locking up (although pages rendered quickly). Key commands that were quirky and decidedly non-Macish. Mouse buttons didn't respond as expected (even my multibutton mouse with scrollwheel behaved more reliably under Safari than Firefox).
It felt like Firefox would be just as comfortable running on an X11 server as on my Mac - but that glosses over the nicenesses of the Mac UI. It just got to be too much. I think the only thing I'll really miss (although that might be stretching it a little) is the WYSIWYG editor support. If I ever really need that, then Firefox is a short command+space Firefox away...
Also, I realized that Safari (with Stand and a few bookmarklets installed) offers about the same functionality in a nice, fast, clean app. I'll keep Firefox around for testing and debugging stuff, but will be using Safari as my default browser. Or, perhaps, a recent beta of OmniWeb... Damned novelty addiction!