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.
Note: When you start typing in your login information, the warning message can obscure the password entry box. You can press the EnterReturn key after you type in your username (or click outside of the password area) to dismiss the warning.
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.
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.
For developers looking 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.