When entering your password, pay attention to the "Num lock" and "Caps lock" buttons. Numlock can disable the number pad , which will then ignore keypad numerals and Caps Lock changes letters to all CAPS. If you cannot recall your master password, there is no method of recovery. This is a security feature to prevent unauthorized users from obtaining access to your logins.

It is possible to RESET your Firefox Master Password, but you will LOSE all existing saved logins. How to reset Master Password

Hi kevinraymond93, a couple other notes:

(1) The Master Password is a local setting. If you have different Firefox installation that share data using Sync, each one might have a different Master Password.

(2) Firefox 76 has a new feature for users who have NOT set a Master Password, which is having to enter their Windows or MacOS login to view or copy the text of passwords. More info in this article: Firefox asks for authentication (password, voice, face or fingerprint) when accessing passwords. As you can see from the screenshot there, it looks very different from the classic Master Password prompt.