The Master Password protects the Password Manager.

It is recommended that you use a different password for each website. In this way, if anyone gets access to one website, that is all they will have.

The MP is only used to encrypt and decrypt username and password data stored in logins.json (password database) and shown in the Password Manager. Firefox will prompt to enter the MP when it needs to access the Password Manager to fill a saved name and password in a form on a web page or possibly when prompting to save a new name. You should never use the same password on more than one website (domain).

Note that Firefox 70 comes with a builtin password generator that can used to generate a strong password.

See also:

• https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-and-import
• https://support.mozilla.org/en-US/kb/use-master-password-protect-stored-logins