At work I can only use Firefox or IE so no brainer there - I use Firefox. At home I use Firefox but generally have Chrome open to play the odd Facebook game. The thing that puzzles me is I really don't notice any difference in performance between Chrome and Firefox and never have. I have a 4mb internet connection at the moment but noticed nothing different when I had 8.
The only reason I use Chrome is to play around with any HTML5 stuff and simple curiosity. With Firefox's huge add on libraries, Firebug, Tab controls etc I honestly cannot see why so much fuss is made about Chrome!