Feature helps defend users against certain kinds of man-in-the-middle attacks. Feature is on by default in the just-released Firefox 23. It s the term used to describe insecure content such as images, audio, video, JavaScript, JavaScript and CSS that are served on HTTPS pages. Users can decide to block Mixed Passive Content as well by following a couple simple steps, Mozilla wrote. The change will present itself in the form of a shield displayed in the address bar whenever she visits a site with mixed content.
Source: https://threatpost.com/mixed-content-blocking-appears-in-firefox-23/101718/