1: Press Windows + R (or go to Start->Run) - This will bring up the Run menu:
and paste in "%APPDATA%\Mozilla\Firefox\Profiles"
2: Below is the screen that will open up find your profile folder which by default should look something like xxxxxxxx.default.
3: Inside your profile folder, find the file called persdict.dat and open it up in your text editor of choice.
Just right click on it and select "open with" from the menu that opens:
Here I've used Notepad to open it up
4: Now just find the misspelled word that you added and delete the word then save the file and you're done.
It is important that you have Firefox closed when you save the file.
Totorial written Blowmax10