Recently Firefox version 23 introduced blocking of “Mixed Content,” something Google Chrome rolled out last fall. “Mixed content” is when information from a non-secure site (without a security certificate–and uses “http” in the URL rather than “https”) is displayed on a webpage on a secure server, within a learning management system for example. The video below shows how to allow that content to display in FireFox. Unlike Chrome, however, the approval is on a page-by-page basis rather than approving the entire site. That means each page that loads with mixed content needs to be approved.
This content blocking often has peculiar indicators. If a video is being blocked, it could display a warning that there is an issue with Adobe Flash. While there could be an issue with Flash, check to see if the content is being blocked as indicated in this video, especially if you have recently updated your flash player.
Desire2Learn users should follow this best practice post to avoid having this happen to content in their courses.