IE is proprietary software, meaning Microsoft owns it. Which means Microsoft writes it.
Firefox is open-source, meaning any smart hacker (programmer, not cracker) who has an idea how to make it better, can.
So naturally, it'll be better. Plus, sweet extensions. I haven's seen one of those 30 second "online episode sponsored by" ads in ages.
EDIT: Not to be rude...but it is in no way a matter of preference, Firefox is a recompilation of Mozilla's Netscape...which, in it's day, was supreme.
In this way, it's more experienced than IE, and it's actually secure, something IE simply is not (which is why they just released, what is it now, version 8?)