Are you sure the protected mode is disabled? Firefox has a hidden setting for this now.

  1. In a new tab, type or paste about:config in the address bar and press "Enter". Click the button promising to be careful.
  2. In the search box above the list, type or paste "flash" and pause while the list is filtered.
  3. Double-click the "dom.ipc.plugins.flash.disable-protected-mode" preference to switch its value from "false" to "true".

This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox and try again.

Note: Please make sure that your Video drivers and Flash plug in are up to date.