Open the Firefox web browser.

   Load about:support.
   Click on the "open folder" link near the top of the page that opens; this       opens the profile folder.
   Close Firefox.
   Check if you see a file called logins.json.corrupt.
   If you do, rename the file to logins.json to fix it.
   Start Firefox.

I have the same problem, however it's not the "corrupt file" issue, it seems on the refresh firefox switched from Password Manager to Lockwise without transfering the data, I have the key4.db and the login.json files in my profile but Lockwise tells me there are no password. Any help for that problem?

I could solve it by myself but I'm unable to find that button again ;( I had in the menus somehow to activate password backup and in a second the passwords did show up!!

I think it was when I switched to the menu entry Logins and password and then I had on top right the entry backup passwords.

At least for me the issue is solved.