Do you ever use the spell checker option on Firefox to replace the misspelled word with the correct spelling? Do you ever screw up and accidentally add it to your dictionary? I do all the time and for awhile I felt like I could not trust my dictionary because I had tainted it. So after searching the internet here is a solution to remove those pesky misspelled words from your dictionary
1. Open your application data folder. On XP or Vista, go to your Start menu and hit Run (or just press Windows-R) and paste in %APPDATA%\Mozilla\Firefox\Profiles. on your Mac, navigate to ~/Library/Application Support/Firefox/Profiles. On Linux navigate to ~/.mozilla/firefox/
2. 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.
4. Now just find the misspelling, delete it, and you're done.