The Mixed Content Blocker blocks certain HTTP requests on HTTPS pages. The browser security community divides mixed content into two categories; Mixed Active Content and Mixed Passive Content. This type of mixed content can alter the behavior of an HTTPS page and potentially steal sensitive data from the user. Firefox 23 is now blocked by default in Firefox with the help of the Mixed Content blocker. Users can decide to block Mixed Passive content as well by following a couple simple steps[2] To avoid generating a security warning websites, websites will begin removing Mixed Mixed Content from their HTTPS sites by replacing images and videos with their HTTPS versions.”]
Source: https://blog.mozilla.org/tanvi/2013/04/10/mixed-content-blocking-enabled-in-firefox-23/