In Windows (mostly Vista, a little Win7, rarely XP) IE 8 as I'm most used to it and like a lot of the new features. I do occasionally use FF 3.5 for sites that don't work well in IE (some developers would rather die than support IE). I tried out Safari just cause of the nifty animations, but I don't really use it at all.
When I occassionally play around with Linux or when I'm forced to use a Mac, I use FF.
When I occassionally play around with Linux or when I'm forced to use a Mac, I use FF.