Password autofill not working, even when "Autofill logins and passwords" is set on
I had to clear out my saved cache recently due to the fact that a certain website would not work any time cookies were made for it, and clearing out my browser's chache was the only solution i found. I already assumed it would remove all of the autofill, and maybe even my saved passwords from the memory (thankfully it didn't delete the passwords themselves), and I thought I would just be able to log back into everything and have it autofill from there. Even with the option turned on in settings, it still doesn't automatically log me in when i load up the page, the autofill is there when i go to manually log in, but for something like youtube it can get irritating, even if it is 3 button clicks.
So, I went into my preferences, and 'Remember logins & passwords' under 'Privacy & Security' was unchecked for some reason.
So, I rechecked it, cleared data under the same section (you will get a notification that you'll be logged out of all sites). Then I quit Firefox and restarted, and my passwords were auto-filling again.
Hope this will help you.