Mozilla on MacOS X is nice too!

If you leave the "Classic" theme (which I always change, except in this case), it emulates the MacOS X look. Of course, the menus and everything are actual Cocoa. Very nice.
It's actually pretty zippy on my system - but then again, my system is a Dual 800MHz PowerMac with 1.152 GB of RAM. So your mileage may vary.