I'm guessing that it's Adobe Flash Player Plugin which you need. It's probably already on your PC but not working properly. If so, Firefox will tell you that it's not present but (because it's already there) any attempt to download it will fail. This is a common problem; people usually notice it when they find that they can't view Youtube videos.
Go to Start > Control Panel and double-click on 'Add or Remove Programs'. Remove Adobe Flash Player Plugin. Then go here, de-select 'Free McAfee Security Scan Plus' and click 'Agree and install now':
http://get.adobe.com/flashplayer/
Chris