For me it depends on what I am doing.
For buying things, research, business - IE.
For general browsing, forums, downloads - Firefox.
The way I see it is when I am operating under my 'sensible business' persona, I use the Microsoft offering; and for when I revert to a 'normal person', I want the livelier Mozilla browser. The other advantage of using two browsers, is that you can run both side-by-side without having to reorder the tabs to accommodate what you are doing at the specific moment - Also with 2 monitors, it works so well.
I have tried most browsers which have been released; I used Opera for a while, but in the end I decided upon IE and FF - No surprise as I see these as the two best general browsers available.