I added a word to my Mozilla Firefox spell check personal dictionary by mistake the other day, and could not figure out how to remove it. Firefox seriously lacks custom dictionary management.
To open your profile directory do the following or use the “Open Profile Folder Firefox Add-on“…
Simply go to Start -> Run, enter:
Locate the profile you’re using (usually default.something).
Then, inside the folder you will find a file named “persdict.dat”.
Open that, and there you have it, your custom list of words.
Edit it how you like save, and restart Firefox.