Have you seen this:
support.mozilla.org/.../control-firefox-automatically-fills-in-forms

I routinely fill in an entire form, about a dozen fields, with about three
keystrokes, and would expect it to work with larger forms.

See also:
support.mozilla.org/.../wont-remember-addresses-phone-numbers-text

There's also this:
https://addons.mozilla.org/en-us/firefox/addon/autofill-forms/
but I have never needed it.