What about the rest of your profile? Settings, bookmarks, history, passwords . . . .

Firefox 80+ has a logins-backup.json file in the profile folder that can be used in case of issues with logins.json or to undo an accidental delete of a login, so you can check if you can find this file in the profile folder or in a backup and replace the current logins.json. Make sure to keep a backup copy of all involved files (logins.json and key4.db and logins-backup.json)

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.

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

If you use a Primary Password then you can prevent losing passwords by canceling the PP prompt. You need to logout beforehand by canceling a PP prompt, e.g. by clicking the eye icon in Lockwise (about:logins).

In a Private Message, chris431 said;

Hi Fred, Thanks for sending me the message about my disappeared passwords. I have followed your instructions and opened the logins.json file and can see the sites that I have previously stored passwords for. However, the passwords in the json file are encrypted, not in the ordinary text I used to create them. Thus I don't know what they are. What can I do with the json file either to return them to automatic use, or to be able to read the passwords so that I can go to the various sites and restore them for future use? Cheers Chris

Re-read my last post. You are looking for the logins-backup.json file. Follow the instructions above.

I followed your instructions but there was only the logins.json file, no logins-backup.json file. I've attached a screengrab of the files that I could see.

Firefox (or another utility) can only decrypt the encrypted usernames and passwords stored in logins.json if you have a matching key4.db file. If the logins.json file is valid JSON (i.e. you can open logins.json i a Firefox tab and the JSON viewer doesn't complain) then this can only mean that your key4.db is corrupted or isn't a matching key file.

So does that mean nothing can be done?