Try to disable hardware acceleration in Firefox. *Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available" *https://hacks.mozilla.org/2010/09/hardware-acceleration/ *https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers Try to "disable the hardware acceleration" in the Flash Player. *http://helpx.adobe.com/flash-player/kb/video-playback-issues.html See also: *https://support.mozilla.org/kb/keep-flash-up-to-date-and-troubleshoot-problems Flash "Display settings" window: *http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html

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?]