Mozilla Firefox includes a spell checker and like any such function, it offers a chance to add words to a custom dictionary. Of course, you can also add misspellings too and these definitely need to be removed. With Word, it’s a matter of looking for custom.dic and deleting the nefarious item. With Firefox, it’s similar, at least on Windows anyway. The file that need to edit is persdict.dat and you’ll find it in C:\Documents and Settings\[user name]\Application Data\Mozilla\Firefox\Profiles\[random name].default. My search for the relevant information took me over to Lifehacker.
Update 2012-12-11: For users of Linux, the location of the above file is as follows: /home/[user id]/.mozilla/firefox/[random name].default. Once you find persdict.dat in there, the required editing can be perfomed.