On the PC I use Firefox almost exclusively, only opening Internet Explorer for pages which don't work with Firefox, which is very few these days.
On the Mac I am still undecided. When I first got my MacBook I downloaded FF immediately expecting much the same as the PC version, but I was a bit disappointed. FF was slow to load up, the interface looked a bit Windows-ish (ugh), and it crashes regularly on MySpace (this probably has something to do with peoples poor coding. I tried Opera, Camino, Safari and OmniWeb and settled on Camino for about a month. However, there were some things such as flash sites which Camino has trouble displaying and it was just getting annoying, so recently I have moved over to Safari.
Although, I am not to sure about Safari now...on the university network it keeps losing its connection. This maybe the uni network, or it maybe Safari, I don't know yet.
So Safari for now. Also, I love Safari's smooth scrolling, FF's scrolling just annoys me!