Firefox for its customizability. If Chrome had as extensive and useful a collection of add-ons, I might use it for for my primary, but right now it's a distant second.
I'm still using Internet Explorer... I know! I'm in the olden days with horses and where the west was still wild with six shooters and whatnot.. but it's what I know and what I can use the fastest.
I prefer Firefox, but I also like Google chrome's features too. I and my family dislike Internet Explorer so much that we nicknamed it Internet Exploder because it always crashed on us.