I always use the spell checker, i don't make many typos, but there are times when i'm spelling a word by how i think it sounds or my hands are just plain dumb and don't spell right, like i'd spell 'misspell' as 'mispell', but i don't use the spell checker, i find it easier to hit backspace then respell the word, if it's one of those longer or oddly spelled words or something, then i'd need to use the spell checker
Also, when ever i say "i'm", "i'd", or "i'll", Firefox underlines those, cause i'm suppose to capitalize the I's when i refer to myself, but i don't, cause i'm lazy :D