Firefox spellcheck - I don't spell that way! Help!!!

I love spellcheck. I usually use it mostly as a typo-check, since I'm usually a one- or two-fingered typist. Most things i know how to spell, my fingers just can't keep up with my brain. I have some complaints about FF's spellcheck. It says that bu and i are words; if i type bu, i was trying to type but, and I always try to use I not i, but sometimes my pinky doesn't make it over to the shift key fast enough. I've added coreopsis and daylily to the dictionary, as well as wintersowing. But is there a way I can take out the words I'll never ever use? Will it ever learn that 'but' is my most frequently mistyped word and not 'bu'?

Any help would be appreciated...

xx, Carrie