Some Firefox users on Windows Vista and Windows 7 are experiencing problems when viewing videos in Adobe Flash Player 11.3. This includes me. Flash 11.3 doesn’t load video in Firefox. Firefox and/or Flash Player may crash. Videos do not work correctly, I just get a black box. Adobe and Mozilla are working closely to diagnose and address these issues but there are no ready solution I know from them in time of writing.
I was pretty frustrated with the problems, but finally after some searching I found a solution worked well for me.
How do I troubleshoot Flash Player’s protected mode for Firefox? web page says that starting with Flash Player 11.3, a new protected mode was added as a new security enhancement for Firefox on Windows Vista and higher. This new protected mode has caused problems to many users. The web page gave one tip that worked very well for me: disable Protected Mode.
To disable Protected Mode, add the following line to your mms.cfg file located in C:windowssystem32macromedflash or C:windowssyswow64macromedflash directory:
To make the mms.cfg file editing to work I needed to first save the mms.cfg file to a writable location (my home directory). I also saved a copy of original file for backup purposes. Then I edited the copied version of mms.cfg file to have ProtectedMode=0 on the last line after other settings. To make changes to work I copied the edited mms.cfg file into the original folder using Windows Explorer.
After I did the changes and started Firefox, everything Flash related worked well on Firefox (as they worked before Flash 11.3 updare).
This should be a temporary fix. Adobe recommends that Protected Mode is manually enabled once the issues with it and Firefox are resolved. To re-enable Protected Mode, the web page says that you simply remove the line from the mms.cfg file using the same technique.