Do you mean a dictionary that you installed from the Add-ons site?

https://addons.mozilla.org/firefox/language-tools/

I'm not sure where those go, but most likely your Firefox profile folder, or a shared location nearby (i.e., up the folder hierarchy under Firefox somewhere). See: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

Or if you meant your personal dictionary (i.e., words you want Firefox to consider correct even if they are not in a dictionary): the persdict.dat file is stored in your Firefox profile folder.

Well, I installed a French dictionary and it was saved as an Extension and stored in my profile folder under Extensions.

You can manage extensions on the Add-ons page to see if your dictionary is there, possibly disabled. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Dictionaries. Then you can review the list on the right side.

If the left column does not have a Dictionaries category, then you do not have any add-on dictionaries installed.

The persdict.dat file is the one I was looking for. But I checked my Profile and the only persdict file I found had very few entries. Some years ago I was able to see many entries and delete the ones that I had added incorrectly. But not now.

I'll keep looking though.

Thanks for your fast response and time.