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 changing  security.insecure_field_warning.contextual.enabled to true and signon.autofillForms.http to false.

firefox-52-contextual-warning-insecure-password-field

Firefox 51: Mozilla turns on Insecure Password Warning