We’ve told Firefox will warn you in the URL bar mentioning the connection is not secure when you visit an insecure page that has login form. Firefox 52 to show the contextual warning to users right in login fields on an HTTP page or form if you try to type username or password : ‘This connection is not secure. Logins entered here could be compromised. This feature is currently disabled by default, you can test this by changing security.insecure_field_warning.contextual.enabled to true and signon.autofillForms.http to false.
Update: If you don’t want to see the warning in login fields, change security.insecure_field_warning.contextual.enabled preference value to false. Then you can still see the warning when you click the lock in URL bar, but not in username /password field.
And for Autofilling in forms to work you need to change ignon.autofillForms.http preference to true.