In your profile directory, the file is persdict.dat and you should know how to get to your profile. One way is to enter about:support in the location bar then click on "Open Containing Folder". Or go through the Help menu > Troubleshooting Information > ...

You can edit with wordpad, or notepad++, but not with notepad, because there is one word per line and you will not see that in notepad after Firefox has updated it.  Update after closing Firefox.

I will have to change my Spell Check documentation to not suggest notepad

http://dmcritchie.mvps.org/firefox/tab_capacity/001_with_underscore.htm#spellcheck

Thanks, I think we got it! How could anyone be able to find that alone? You are the greatest.....

See also:

• https://support.mozilla.com/en-US/kb/Using%20the%20spell%20checker#w_removing-a-word-you-have-accidentally-added

You might note that persdict.dat in your profile is used to spell check all the languages you have installed. So deleting a word in the current language will remove it from any other language you use.

(Guess this is a good topic for a Request for Feature: - 'separate persdict for each language dictionary.)

@edb_gene

It would be better if Firefox had a user interface to access that file and edit it, instead of the user having to dig around the "bowels" of the operating system looking for a folderset and file that are hidden by default, at least on Windows.

If you are serious about doing a Request for Enhancement, you can do that at Bugzilla - https://bugzilla.mozilla.org/ - but do a thorough search for an existing Bug like that, first. If you find an existing Bug, you can vote for it. If not, make sure you read the Bug writing guidelnes page first (I filed a few Bugs in my 9 years using Firefox and a few weren't taken seriously due to mistakes I made in the report) and reading the Bugzilla Etiquette page. Honestly though, I would be surprised in something like that was added. It has been like this for over 6 years now and I can't recall anyone else ever mentioning a separate personal dictionary for every installed dictionary - although that would make for a nice feature, IMHO.

Before Firefox had a built-in "spell check" feature (as of Firefox 1.5), we used an add-on named SpellBound - a version of which was pulled from the "trash heap" and made compatible with Firefox 3.0 - 3.6 - https://addons.mozilla.org/en-us/firefox/addon/spellbound/ - but it looks like it has been abandoned again. That extension had a lot of good features, like an editor for the Personal Dictionary file from with a UI, which were not included in Firefox's new "spell check" - well that does happen sometimes. But the "killer" is that the same person who developed the original version of SpellBound - http://spellbound.sourceforge.net/ - was hired by Mozilla and he coded the new built-in feature. I figure it must have "killed him" to have to leave out the best features that he had in his extension, but it does now work "inline" while the user is typing, rather than having to be opened to check all the spelling in a text-area all at once, as was the case when it was "added on" to Firefox.

@the-edmeister Thanks for the tips (I'm almost on my way over to Bugzilla).

The point I was making here is that this support page should (IMHO) warn the user that the editing affects ALL languages.

(That is: 'There is only one persdict.dat'. If you are using multiple languages with something like Quick Locale Switcher by Captain Caveman, any editing will be reflected in all the locales.)