clipped from: ffextensionguru.wordpress.com   

The built-in Spellcheck with Firefox 2 is great, but it is very easy to accidentally add a word to the dictionary that you didn’t want to. However, it is not so easy to remove the word or words added in error. Actually it is easy to do just not really documented anywhere. After looking around on Google to help a Go Firefox! forum member with this issue, I found the process:

  • Close Firefox (Firefox saves the dictionary file upon closing)
  • Navigate to your profile folder (Locate your profile folder)
  • Look for the file persdict.dat
  • Open with a text-editor
    • Notepad is NOT recommended as it removes all the line breaks between words. OpenOffice Writer works well.
  • Find the misspelled word or words and remove.
  • Save the file and restart Firefox