As you know, when you set a master password, you usually need to enter it before you can view or have Firefox fill in saved passwords. See: Use a Primary Password to protect stored logins and passwords.

This has to be done once during a session, so someone who does not know the master password should not be able to start up Firefox and view the saved passwords.

However, once you have unlocked them in the current session, I am not aware of a quick/convenient way to relock them. I think you would need to exit out of Firefox to do that. Perhaps there is an add-on which allows you to relock them any time on demand.

You can log out of the Software Security Device if you click the "Show Passwords" button in the Password Manager and cancel the request to enter the Master Password.

• Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

Then you can close the Password Manager and you need to re-enter the MP to access the passwords.