In a new tab, type or paste about:config in the address bar and press "Enter"
In the search box above the list, type or paste "flash" and pause while the list is filtered.
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.