The Secure Sockets Layer warning in Firefox informs you of potentially dangerous content on websites. The browser requests the security certificate from the website and checks it to ensure it is valid. If the check fails, Firefox prevents navigation to the page. You can always add an exception for a website if you have the privileges, but you'll have to do this for every exception. It's possible to disable the warning from popping up permanently.
Type "about:config" into your address bar, and press "Enter."
Click "I'll be careful, I promise!"
Double-click "browser.ssl_override_behavior," and change the value to "2." Click "OK."