I use the master password in firefox to protect my logins.
I have noticed that even if I get the password wrong I am able to access sites and have my password supplied by Firefox. I have been using Firefox for years and really like it and have it syncing between my various computers (Desktop, laptop, laptop VM, phone).
Opaite Mbohovái (1)
If you get the Master Password wrong, Firefox's password manager should not fill login forms. If you can get into sites even without entering the Master Password, it's possible a cookie is keeping you logged in. As a check, you could log out of the site, then test again whether Firefox will log you in if you don't correctly enter the Master Password.