Try to rename/remove the secmode.db file in the Firefox profile folder.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

If that doesn't work then rename/remove the logins.json and key3.db files as well. Note that removing these two files will remove all saved passwords in the Password Manager.

I did not understand

This is about replacing files in the profile folder to force Firefox to create fresh files.

Do you have a specific reason to reset the master password (forgot your master password)?

• https://support.mozilla.org/en-US/kb/use-master-password-protect-stored-logins

If you can use the chrome URI to reset the master password then you need to go to the Firefox profile folder and rename or remove the files I posted above. You can start with renaming the secmode.db file if you haven't used the master password before. If that still doesn't allow to access the Password Manager in "Options/Preferences -> Security" then you need to rename the actual files the store the passwords. The key3.db file stores the encryption key including the master password that is used to encrypt and decrypt the usernames and passwords. The logins.json file stores the encrypted passwords.

Easiest to access the Firefox profile folder is via the button on the Help -> Troubleshooting Information page.

• Help -> Troubleshooting Information -> Profile Directory: Show Folder
• https://support.mozilla.org/en-US/kb/Profiles

Thank You !

I understood