Since upgrading my kids' browser to Firefox 4, YouTube seems to have switched them over to HTML5 video, which the computer apparently can't handle because it stutters horribly, causing Child Rage Syndrome. ;-) IE9 does not pose this problem, it uses the Flash player.
I right-clicked one of the offending videos & navigated to http://www.youtube.com/html5. It initially told us we were in the HTML5 trial, and so I clicked to opt out. I then exited the browsers & ran ccleaner to ditch the cached stuff.
No fix. Now the above YouTube page tells us we are not in the HTML5 trial, but we are still getting the laggy, HTML5 video (I can tell based on the right-click menu that comes up). My kids have switched to Internet Explorer, but they miss Firefox (and in particular adblockplus).
Is there a way to get YouTube to really switch us over to flash?