Edit: given how common this problem must be, would suggest:

• a quick access option in settings enabling users to easily edit the personal dictionary
• an option upon addding something to the personal dictionary enabling users to 'undo' the wrong 'correction'

With respect it seems such a very basic feature as modifying one's personal dic is complicated or in this case, not possible.

Do you have more than one profile folder -- so that maybe you need to find the file in a different one? You can check the About Profiles page. Type or paste about:profiles in the address bar and press Enter to load that page.

The profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

The Root Directory line has the button to view that particular profile folder in Windows File Explorer.

Otherwise, hmm, I don't know where else that file would be. ??

Thanks for your reply. Indeed there were two profiles. Removed one. Added a dictionary. Still no sign of persdict.dat.

I can't think of any other place it would be stored. Is this a standard Mozilla installer, or the Microsoft store installer? The Microsoft store creates its profile in an obscure location.

Assuming standard: does it make any difference to turn on viewing of hidden files?

https://support.microsoft.com/en-us/windows/view-hidden-files-and-folders-in-windows-97fbc472-c603-9d90-91d0-1166d1d9f4b5

Thanks again for your reply. I'm fairly sure it was downloaded from Mozilla. Showing hidden files doesn't make any difference. A full search persdic.dat doesn't show up anything.

Unless Firefox is failing to create/save the file, which would mean that your saved entry isn't there after you restart Firefox, it's hard to understand what is going on.

When I web search this, it seems there are occasional reports of the same problem, none of which had a solution. When I look at the source file, it seems that both the file name and the location in the profile folder are hardcoded (https://searchfox.org/mozilla-release/source/extensions/spellcheck/src/mozPersonalDictionary.cpp lines 30 and 205), so... wtf.

Do you use any Add-on dictionaries for other languages/locales?

You did close and restart Firefox ? What happens if you create a persdict.dat file yourself and add some words to it ?

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thanks. Have created persdict.dat myself and it indeed stores misspellings.

Yes I have added one dictionary.

British English Dictionary (Marco Pinto) A fork of Mark Tyndall's add-on, based on David Bartlett's British Dictionary R1.19 for Firefox, Thunderbird and SeaMonkey. V3.1.2 has 130 000+ new words and also fixes. British Dictionary V3 for Firefox, Thunderbird and SeaMonkey.

Thanks for all the replies.