What are the rules for Firefox to auto-fill credentials in a page?

Add-ons Development
1

On some pages, only email is autofilled and password is not - even though there is only password available and it’s offered when I focus the field:

Screenshot 2021-03-12 10.45.53
Screenshot 2021-03-12 10.45.53488×335 47.2 KB

I know I was able to fix this in the past with userScript simply by adding invisible inputs (when there was only email or only password field) which helped Firefox to autofill it.

Can someone tell me what are the exact rules? Does it have to be in a form? Are there some attributes that are being checked (autocomplete, autofill, name, type, etc…)?
I want to know what if Firefox looking for when it’s “thinking about” auto-filling something.

Thanks!

EDIT:
Could this be related to the fact that the whole input field is actually in an iframe? Is this some Firefox protection from password leaking that Apple uses to prevent auto-fill?