Try disabling protected mode for flash:

   type "about:config" in the address bar
   click the "I'll be careful" button
   In the search box, paste "dom.ipc.plugins.flash.disable-protected-mode" (without the quotes)
   double-click to set the item that appears to true 

You may have to restart firefox for this to take effect

You can check for problems with current Shockwave Flash plugin versions and try this:

• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)
• disable hardware acceleration in the Flash plugin

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting