Check the FAQ: https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile#w_passwords

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

key3.db and logins.json : These are the Password files.

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.

With all due respect and please forgive my frustration but I already know that key3.db and logins.json are the password files as you would realise, had my question been read properly.

Let me put the query another way. Which directory do the password files key3.db and logins.json reside in order for Firefox to access them?

I have just managed to retrieved the lost files key3.db and logins.json, which had been deleted during the recent Windows 10 update and I now wish to know to which directory I should copy them, in order that Firefox can access them.

By the way, saying that:-

"In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password."

- is akin to locking the horse after the stable has bolted. A little unhelpful under these circumstances.

Again I apologise for my frustration.

I understand. I have lost my own files, but I had backups to use.

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox. COPY the files here. Then restart Firefox.

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

FEBE (Firefox Environment Backup Extension) {web link} FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backups of files that you choose.

OPIE {web link} Import/Export extension preferences

Thank you Fred, that is just the information I was looking for. Everything is now back in place with 90%+ content reinstated.

I am now almost certain that the loss of data was caused by the recent Windows 10 anniversary update. Not so desperate for me thank goodness, I understand that some have had catastrophic loss, with partitions being trashed, et al.

Thanks again.

Glad to help. I advise you to use the add-ons above to create backups.

BTW, Lots of users complained about Win 10 anniversary.