As always, Calum is on top of his shit. You are better off without IE, as mozilla will most likely function well after IE has crashed & burned on you. And who really only has one browser on their machine? That sounds like a good poll, "How many browsers do you have installed on your computer?".
When IE at work fails, as it is prone to do, i use mozilla to get the fixes & patches, so Open Source saves the day for my proprietary ms crap.