1. Firefox + flashblock ( this add on have a whitelist control)
2. Chrome , google has already force chrome to use HTML5 player on youtube 2 years ago.
If you insist using IE, go to https://www.youtube.com/html5
You should be able to request/switch using HTML5 or Flash as default player on youtube
However, some of youtube video was encoded in WebM VP8/9, I think they cannot play by IE via HTML5 natively.
(Not very sure since I haven't use IE for so many years)
you may need to install WebM Video component for IE https://tools.google.com/dlpage/webmmf