I have Firefox 3 and Flock 2 installed in addition to Safari 3.2. Flock is my default as I like the integration with myspace, gmail, flickr, picasa, del.icio.us, Yahoo, etc and the integrated rss feeds sidebar and media upload and viewing. Plus just about every extension compatible with Firefox 3 will work with Flock 2 ( I haven't found any that haven't). When I was still on my aging pc it was a lil slow, but so was Firefox but that was mainly due to me liking to have 10-20 tabs open for weeks at a time.
At my last job I also had Camino installed but I didn't bother with it after I bought my macbook pro in october.
I almost never use Firefox anymore. Its usually Flock plus Safari here and there.
Like others have said its a personal preference. Likely to erupt into a flame war almost as quick as whats the best OS. At least there's no IE on the Mac anymore 🙂