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if you actually use everything in the mozilla suite, then its awsome. if you dont, then its alot of overhead for stuff you just dont use. also, the ui on firefox is extremely well thought out and designed. not to mention that there are many people who were around during the "browser wars", and have very strong emotional responses to anything netscape.
anyways, theres many reasons, some good, some not so good.
not really sure how moz can rendor a page correctly, and ff bork it, as they use the same rendoring engine. is there anyone here who can answer that? does ff use a modified gecko, or is it pretty much the same thing as in seamonkey?