Firefox will display a lock icon with red strike-through in the address bar, when a login page you’re viewing does not have a secure connection. If you enter a password on such a page, eavesdroppers or attackers could steal it.
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 the login page is insecure, check if a secure version exists by adding https:// in front of the website address. You can also contact the website administrator and ask them to secure the connection.
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 gray lock icon"HTTPS" in the address bar, along with a lock icon . Pages that don’t transmit any private information can have an unencrypted connection (HTTP). But, it is advised not to enter private information, such as passwords. The information you enter can be stolen over this insecure connection.