I agree, Maxthon is the best IE-based browser, but it doesn't compare with FireFox. You obviously didn't use Firefox for enough time to see its full potential...
First of all, Mozilla Firefox uses a better rendering engine, Gecko, and it's much more secure than IE (Maxthon has most of IE security bugs). But the best thing about Firefox is the use of extensions (and themes)... You can make Firefox act and look the way you like it (even make it work like Maxthon). I tried many browsers (Maxthon, Opera, Avant...) and every browser has some good things, but I can say that Firefox has it all... No surprise that Firefox reached 1 MILLION DOWNLOADS in 100 hours.
And rememeber, Firefox is still BETA...