My main reason for using Firefox is the "FlashGot" extension which downloads all (or some) of the files on a page with just one keystroke - it even starts downloading in the background as soon as a page opens (it uses the free included UNIX downloader cURL) It's an ideal download compagnon for AllOfMP3.com

Safari still remains my default browser, because:

1) Firefox is incompatile with the keychain
2) it also doesn't support Apple's API for contextual menu plug-ins, meaning that for exemple URLManagerPro's "add bookmark" plugin doesn't show up.
3) it takes too long to launch.

A good thing though that you can change the ugly PC-style interface with a nice theme, like GrApple, and you can also change the PC-style buttons with mac-style widgets using Firefoxy.