I normally use X-Notifier Reloaded with Firefox 56. When I start the browser, it asks for my "Master Password". If I enter it, X-Notifier works fine, if I don't then X-Notifier does not see any of my email accounts and doesn't work until I reload the browser and enter my Master Password.
XN 4.x
Master Passoword is specifically to protect website logins.
If I remember correctly, I think the changes to Firefox that forced creation of X-Notifier 4.x... also resulted in not being able to use Master Password the same way for X-Notifier.