I've had the opportunity to work with the safari team in the past. I can't say a lot due to an NDA, but lets just say there is a good reason to prefer Firefox.
I use multiple OS's throughout the day. Firefox works on all OS's smoothly. Safari doesn't let you set a custom search engine. Firefox has some great extensions.
I'm currently a Firefox user at home and work, but thinking about going back to Safari in the near future...