Using the Master Password feature, you can protect all of your locally stored usernames and passwords with a master password. If you have forgotten your master password, you must reset it.

Warning: Resetting your master password will remove all of your saved usernames and passwords.

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1. In the Firefox location bar, enter the following location:

chrome://pippki/content/resetpassword.xul

2. Press Enter.

3. The “Reset Master Password” page will appear. Click the Reset button, to reset your master password. 

Remember: Resetting your master password will remove all of your saved usernames and passwords.

Via Mozilla