Firefox 32 and later builds store the passwords in the logins.json file in the profile folder and no longer use the signons.sqlite file. You can force Firefox to reimport the passwords from the signons.sqlite file. • reset the signon.importedFromSqlite pref on the about:config page to the default value via the right-click context menu • delete the logins.json file in the Firefox profile folder with Firefox closed

When you restart Firefox then you should have the signon.importedFromSqlite pref with the value set to true and you should have the passwords imported in the Password Manager unless there may have been errors.

You can use this button to go to the currently used Firefox profile folder: Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

The file is in object format.

You can try to open the file in the Scratchpad (Firefox/Tools > Web Developer) and click the "Pretty Print" button to format the file for easy reading. It basically consists of two sections: logins and disabledHosts (never remember). You would have to check the file for anomalies. Note that the encrypted names and passwords can only be decrypted if you have the correct key3.db file.

{
  'nextId': xxx,
  'logins': [
    {
     'id': xx,
    },
    {
     'id': xx,
    }
  ],
  'disabledHosts': [
  ],
  'version': 1
}

It fails to open file. Nothing displayed. I copy pasted from notepad++ however it shows weird characters and at the end i see these

Exception: Unterminated string constant (1:49146)

Sounds that the file is corrupted.

Do you have an older copy of the signons.sqlite file that you could possibly use to rebuild the logins.json file (you will lose passwords created with the logins.json file)?

Is there binary data present in the file suggesting a problem with the cluster chain? Did you run the chkdsk.exe program in a cmd.exe window?

You can try to compare the last part with a working logins.json file to see if you can find out what to need to add to fix that last part and remove the last incomplete password record within {} and add closing brackets }]}.