I can tell you what I prefer, but not what is better.
Prior to swapping in a faster HD and more ram, I preferred using Safari because it booted up faster, handled a lot of tabs at once better (for me), and was less of a resource hog (for me). If I left Safari on for extended periods of time however (especially if one of those tabs had video on it...), it would bog down my entire system. The one thing that I really like that Safari has over Firefox (natively, and at least to my knowledge) is top sites. I've taken a great liking to top sites, and now only using bookmarks for archiving references.
That being said, I always kept Firefox around because it handled certain sites better (particularly banking, for me) and every now and then, I would get sick of being bombarded with these intrusive ads (which are the #1 cause of app crashes on my mac by far). I'm aware of glimmerblocker and all that jazz, and even it installed, but it doesn't block a lot of stuff (for me) without making adjustments, which are easier (for me) to do in Firefox with Adblocker.
This changed with my faster HD and more ram though, now the performance differences between the two browsers are virtually negligible to me, but I still primarily use Safari because of top sites alone. If Firefox ever included a native top site-esqe feature in their app, I would probably switch to it full-time.