If you haven't made any other changes in the Password Manager then the old login might be saved in logins-backup.json. 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/update of a password (only the last edit is saved). You can possibly compare logins.json and logins-backup.json to see if they differ for that specific origin (protocol and hostname). The usernames and passwords stored in a logins file are encrypted, so you would have to replace logins.json by logins-backup.json temporarily with Firefox closed. Make sure to keep a backup of all three files logins.json and logins-backup.json and key4.db (encryption key). 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 <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data