I came up against the problem this afternoon. The following support page is relevant : Insecure password warning in Firefox.
Firefox will display a lock icon with red strike-through red strikethrough icon in the address bar when a login page you’re viewing does not have a secure connection. This is to inform you that if you enter your password, it could be stolen by eavesdroppers and attackers.
Starting in Firefox version 52, you will also see a warning message when you click inside the login box to enter a username or password.