So currently, you can set a PIN code in the Firefox extension, so that you can hide your passwords. But when you go to a website, for that you saved a password, you can simply view the password from the autofill suggestions.
The password should stay hidden here and nobody should be able to view the passwords from there.
(If it depends on the website whether entered passwords can be viewed, an option would be to detect that and/or ask for the extra/master password before autofill)

In my opinion that would be the most basic function of any password manager. Nobody, i.e. neither people who happen to stop by at my computer, nor people who also use my computer should be able to get my passwords presented immediately...