This is a new feature that is available starting in Firefox version 51.
Firefox will display a grey lock icon with a red strike-through 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.
What can I do if a login page is insecure?
If a login page for your favorite site is insecure, you can try and see if a secure version of the page exists by typing https:// before the url in the location bar. You can also try to contact the web administrator for the site and ask them to secure their connection.
Not recommended: You can also continue to log in to the website even if the connection is insecure, but do so at your own risk. If you do go this route, try to use a unique password or a password that you don’t also use for other important sites.
About insecure pages
Pages that need to transmit private information, such as credit cards, personal information and passwords, need to have a secure connection to help prevent attackers from stealing your information. (Tip: A secure connection will have “HTTPS” in the address bar, along with a green lock icon.)
Pages that don’t transmit any private information can have an unencrypted connection (HTTP). It is not advised to enter private information, such as passwords, on a web page that shows HTTP in the address bar. The information you enter can be stolen over this insecure connection.
To Learn more about this warning, please see this page. The page explains when and why Firefox shows this warning, and will also provide some details on how to fix the issue. For more information, see this blog post and this Site Compatibility document.