IE is crap. Can't think of any good points either for it!
Firefox does everything I need it to do especially for website development / checking / coding / FTP, cool add-ons / extensions, skins, and a great deal more. Always reliable and quick.
I did try Safari (XP) out of curiosity for webpage speed and had to say it WAS quicker than FF2 but after also trying FF3 certainly looks to blow away all other browsers for speed and reliability.
As for FF eating memory, only time I notice this is when the wife is doing work with Freecycle (massive global websites) and the FF extension loads the database everytime slowing things down... so I just disable this, and any other I know I might not need depending on what I'm doing.