Firefox, if you didn’t know, has a built in spell-check function. (And judging by some of your comments, not all of our readers were aware of this fact.)

Having built-in spell check is great, but I have on several occasions accidentally clicked “add to dictionary” when I meant to select one of the correctly spelled options.

Never Fear, you can remove them.

Here’s how:

1. Go to start–> run and paste the following into the run window: %APPDATA%\Mozilla\Firefox\Profiles
2. Hit Enter
3. Open the .default folder
4. Use Notepad to open persdict.dat
5. Delete the misspelled word
6. Save the file
7. Restart Firefox, and you’re all done

Source: This Site