jason: microsoft have copied a lot of firefox's features in IE7, but it's still a world away. The standards support in IE7 is still below par (firefox and opera are streets ahead).
firefox is likely to be ahead for a while, because it's free software and so anyone can develop it. any security issues it faces are crushed almost immediately, unlike in IE.
I'd agree with the above in that firefox is the best for windows. if you're using some flavour of GNU/Linux then firefox is also currently the best. Mac OS X is a bit of a strange one. firefox does run on it, and it runs quite well, if a bit slow. however, because it's not written in the native cocoa framework like other Mac OS X apps, it doesn't fit in with the rest of the OS quite as well. Safari is a good browser and, as you may imagine, fits in very well with the OS. Camino is a browser written by the mozilla group (same as firefox), but written to fit in more with Mac OS X. Another good browser for Mac OS X is omniweb.