Have you ever mistakenly added a misspelled word to the Firefox custom dictionary. It has happened to me a few times, and I always google to find the answer for editing the dictionary entries.
Look for the “persdict.dat” file in your Firefox profile folder on your hard drive.
Users/username/Library/Application Support/Firefox/Profiles/profilename.default/persdict.dat
Found this forum post and I wanted to blog it so I remember it for next time!
http://www.trap17.com/forums/firefox-dictionary-word-edit-t48009.html
also: Mozilla Support
Hmm, now if I can just remember that I blogged it, we will be all set!