If you're experiencing difficulties watching streamed videos with Firefox it's almost certainly due to the Adobe Flash Plug-in. If it's not functioning properly (as sometimes happens) any attempt to download it fails because your PC thinks that it's already there.
However the solution is easy. Go to Start > Control Panel > Programs > Programs and Features. Uninstall the Adobe Flash Player Plug-in. Restart your PC. (That might not strictly be necessary but I'd advise it anyway). Then download Flash Player from here:
http://get.adobe.com/flashplayer/
Chris