I have been using SMPlayer to watch Youtube videos. Missing was a convenient Firefox context menu option to open links directly in SMPlayer. I resolved the missing feature with the SMPlayer View add-on.
The add-on must be manually configured to find /usr/bin/smplayer.
With this combination I haven’t used the Flash plugin in quite a while to view youtube videos. I also do not need to worry about HTML5 not yet supporting click-to-play.
I only wish SMPlayer directly supported other video sites.