I've tested the video in safemode in both flash and html5 versions through firefox and they run exactly the same. While this is with chrome, which is again the same video: That means faster and light-weighted browser with. These are the choices I get with the same video playing in youtube using html5: The biggest advantage is that you dont need to install adobe flash player or Microsoft Silverlight plugin. Videos only have 360p and 720p as quality options. Now I would just switch over to html5 youtube from now on, but that's the major problem I'm having. I know flash itself has severe performance issues and is slowly being phased out. I have a gtx 780, an i5 3570k and a 250gb 840 evo. I have hardware acceleration with flash enabled, and my hardware is not the problem. The problem is I've still been using flash based Youtube in firefox, and when running the 4k video using flash, I get massive slowdown and the video is choppy and unresponsive. It's not the first, as I watched many of the videos that came out when 4k became supported on Youtube, but it's one of the few. MSE & WebM VP9 show it enable im check it about:config section its show it true. did u guys same or only me youtube videos only play mp4 codecsavc1.4d on me. flash was and is a great thing, but with it’s problems it is not a plugin to. after upgrade firefox 43, i notice youtube videos not play vp9 anymore. that is why flash is dying slowly and HTML5 is taking the control over all the internet. despite the efforts of Adobe to secure flash, third party entities and hackers find more and more bugs on it’s security. I've recently come across a 4k quality video on Youtube. (if I can find the video I watched I will add it here).

Firefox add on youtube html5