Safari definitely integrates better with Mac OSX but Firefox has a feature which I could never live without: Add Ons!
I work in a Win Xp environment and the ability to use the same browser cross platform is perfect for me.
I'll give you an example of something that is priceless for my use: I am at work, I find a good webpage that I want to keep for future reference, I add a bookmark. When I get home to my Mac the bookmark is already there on the Bookmarks Bar thanks to the Foxmarks Add-On. I love not having to use a webpage to sync my bookmarks, it does it behind the scenes with no hassle.
This feature alone is enough for me to tolerate the quirks about the "not so great" integration with OS X's native theme and use Firefox as my main browser.
However, I admit that Safari looks and behaves better, it's faster IMO. Just go to gametrailers.com (example) using FF3, choose a QuickTime HD video and try scrolling the window while the video is playing. FF3 totally SUCKS at this 😀