Insecure password warning in Waterfox
If a login page you’re viewing does not have a secure connection, Waterfox will display a lock icon with a red strike-through 
in the address bar. 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. Otherwise, it is not recommended that you submit any information through the page.
About insecure pages
Insecure pages do not provide secure connections for visitors. Instead, they use an unencrypted connection (HTTP) and any information shared with these pages is at risk. 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.
Pages that don’t transmit any private information can have an unencrypted connection. But it is advised not to enter private information, such as passwords. The information you enter can be stolen over this insecure connection.
Note for developers
For developers looking to learn more about this warning, please see this page. The page explains when and why Waterfox shows this warning, and will also provide some details on how to fix the issue. For more information, see this blog post.