Firefox all the way. Except for the pesky pages that won't work with FF, then I use the IE extention. The lack of ads, popups, and spyware cropping up is just plain awesome.
Of course as soon as I open up IE for whatever purpose, there's flashing ads all over the place. Grrr!
I voted IE, I've never had any reason to switch to firefox.
The reasons to switch are all too apparent once you get used to FF.