Recently released beta version added new security feature to Firefox 18 which allows browser to block insecure content from loading over secure connections. This feature is disabled by default, here is how you can enable it. The bug that exists since 12 years for now has been fixed in Firefox 18 beta, you can now prevent insecure content from loading on HTTPS pages in Firefox by toggling values of two preferences in about:config.
All encrypted sites that lets users connect to their websites via secure connection fails as some elements still load from unencrypted http connections.
If you enable the below said preferences Firefox will block the mixed script content those can be images, CSS, that load from http connections which allows you to maintain privacy communication with the website.
To Disable insecure content loading on Secure websites in Firefox 18 Beta
1. Visit about:config in the browser
2. Find the below two preferences and turn their values to True.