I've noticed the PW field is blank on sites I've saved logins for.
Earlier versions of Firefox, clicking in the blank PW box pops up the (saved) login(s) available.
Pick one, then TAB autofills the PW.
Later versions, I find I have to type the first letter of the login to prompt Fx to show the login choices.
Sometimes I have to fill in the entire login, then TAB, and the PW autofills.
Some sites I visit to pay bills, etc. monkey around with the login page between my visits.
Even though the login and PW are saved, Fx doesn't recognize the fields and does not autofill.
HTH