We’ve already covered that Firefox 23 to block Mixed active Content by default. Let me give you a little brief about mixed content.  On HTTPS page when some content is loaded via HTTP connection there are chances for hackers to modify HTTP content even though main page is served via secured connection, here this HTTP content is called as mixed content.

With Mixed Content Blocker turned on by default Firefox 23 blocks this mixed active content automatically. FYI this security feature is not enabled by default in recently released Firefox 21 stable and Firefox 22 beta versions, but you can enable it easily here is how.

Enabling Mixed Content Blocker in Firefox 21 and Firefox 22

1. Visit about:config in the browser address bar

2. Find the preference security.mixed_content.block_active_content and turn its value to true.