Firefox 52 introduced an insecure form warning: whenever a password is requested on a web form through a non-SSL website (i.e. no "https"), Firefox displays a warning popup below the focused field:

While the gesture is appreciated (user awareness and all), it can become annoying. There is a way to turn the warning off through the advanced configuration of Firefox:

• Start up Firefox and enter the following URL in the address bar: about:config
• You'll get a This might void your warranty! page. Click the I accept the risk! button to continue.
• You'll get a list of everything there is to configure in Firefox. In the search bar, paste the following value to quickly locate the correct parameter: security.insecure_field_warning.contextual.enabled
• Double click the setting in the list to set it to "false". You'll notice that the setting will become bold, indicating it has been modified from the default value.

If you also want to restore the autofill functionality (saved login/password is automatically filled in on a non-SSL website), look for the parameter signon.autofillForms.http and double click it to set it to "true", enabling autofill.