Video plays fine here with FF 16.0.2, ABP, Flash 11.4.402.287 running in protected mode in Win 7 Pro 64bit. It is playing the video in HTML5 as I am in the HTML5 trial which you can turn on or off here: »
www.youtube.com/html5When it's playing the video in HTML5 mode it's not even using Flash to play it so that's something you might check.