If you have used Firefox’s built in dictionary, you will know there is a limitation to it. Firefox allows you to easily add words to dictionary, but lacks a user interface for removing words. This can be a problem since the “Add to dictionary” button is directly under the corrected words, providing easy access for adding misspelled words to the dictionary.
Since I frequently add misspelled words, I had to learn the method for removing these words. To remove misspelled words, first close down Firefox then navigate to your profile folder. The folder’s location varies depending on your operating system:
Windows XP
C:\Documents and Settings[User Name]\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default\
Once in your profile folder, look for a file named persdict.dat, and open it with your favorite text editor. Inside you’ll find all of your custom defined words, each on their own line. Just delete the words which don’t belong and save. Firefox will no longer incorrectly identify those misspelled words as correct.
See "Firefox Dictionary & Spell Checker Hacks: Edit Misspelled Words, Merge with Word Dictionary". Basically, you need to manually edit the file named persdict.dat.
