Please make sure your QuickTime player is correctly installed and Firefox is ''not'' running in Safe Mode. Then go to about:plugins and check for "QuickTime Plug-in". If you don't see that plugin, it means your QuickTime setup is broken or missing. In this case we recommend you to uninstall QuickTime, download again latest Quicktime package from Apple website and try to install it again. If the problem doesn't solve with that, you could check QuickTime Alternative, a free alternative to QuickTime which may solve your problem. If nothing of above solved your problem, we need further information about the problem (whether a blank / black frame appears or not where QuickTime should load, a error message displays, Firefox doesn't detect QuickTime plugin...).