The best thing to do is uninstall Flash if you don't need it for any other sites, but if you do need it, I think you can either go into YouTube settings or go to YouTube.com/html5 and change it there.
Idk what it is, I have 2 laptops running windows 7 ultimate, and firefox always ends up running until its using a gig of memory if not more, and also if im watching youtube there is a second program called plugin-container.exe that ends up running another gig of memory if not more. now if i close the process of plugin container flash reboots and my tabs or videos will say "flash crashed, click here to refresh' or something similar. so I know my issue has to do with flash but even if im not running/watching videos and just using multiple tabs and reading news sites (which are flash heavy) or the coli firefox ends up running high and I have to ctrl alt delete out because my cpu starts running super slow, but once i close it its 100% better and running fast again.
I tried using chrome but its the same...its not much of a bother, I consider it just normal usage and its nothing to ctrl alt delete and close firefox or plugin-container (which is a firefox process). hardware acceleration is off too
Firefox is still great
