There are some compatibility issues between the latest Flash player and some Firefox add-ons or settings. Could you see whether anything in this article helps: [[Flash 11.3 doesn't load video in Firefox]]. You might also consider disabling the new protected mode in Flash 11.3, since that feature is currently being debugged. See this support article from Adobe under the heading "Last Resort": [http://forums.adobe.com/message/4468493 Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?]