hii ajinuk You'll need to close Firefox during the operation. The hope is when you re-open Firefox, it will seamlessly use the old files in place of the new empty ones. Here goes:

Open your current Firefox profile folder using either

   "3-bar" menu button > "?" Help > Troubleshooting Information
   (menu bar) Help > Troubleshooting Information
   type or paste about:support in the address bar and press Enter 

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

   "3-bar" menu button > Exit
   (menu bar) File > Exit 

Pause while Firefox finishes its cleanup

Then right-click > Rename logins.json to something like loginsOLD.json. Also right-click > Rename key4.db to something like key4OLD.db. This is in case you need to undo this change.

Copy/paste the logins.json and key4.db files from the old profile folder to the current one.

Start Firefox back up again. Can Firefox find/read your logins?

I closed down Firefox to try as you suggested, then thought I would restart it first to see what would happen. All the passwords are suddenly back! Thanks anyway!

most welcome