That's because firefox is open source and IE is not. That means users can create their own scripts for firefox which you can't do with IE (same situation as Linux vs Windows). The flexibility and ability to customize based on your preferences is the reason FF became so popular.
Actually what made firefox popular was the fact it was light-weight. But currently its just as bloated as IE in most cases and the majority of popular extensions are already included features in Opera.
If I preferred IE (which I don't), I'd still install Firefox in order to use 'Conquer Club - Roberts Mods' (soon to be BOB) with Conquer Club. If you haven't tried it, you must. If you have, how could you go back?
Seriously, hovering over a territory and seeing its neighbors, or hovering over a player and seeing their territories highlighted, makes this a different game. Plus the stats display? Indispensable.