Tu sum up, correct me if I'm wrong - you already have an account here. But you decided to start logging in via Facebook. You clicked button "Log in via Faceboock" you agreed, then you get form with password. Right?
If yes, then you probably are trying to create new account, and not connecting Facebook to the one that you already have.
Flarum will not "fill password for you" because there is no password sent. Facebook doesn't send you password to Flarum. It just confirms, that your account is yours. In this form you have to provide password in case you want to log in traditionally.