I've heard that now (not earlier) Firefox also supports Youtube's 60 fps now. Before it was said only Chrome supports it. (Which in effect is their own browser, no surprise.)
I use(d) Flash Player 15 on Waterfox (64 bit Firefox). I now updated to the current one - version 17. Didn't make any difference. I play various 'high profile' Youtube videos and the stats tell me that it drops tons of frames - precisely 50%. So Youtube is streaming a 60 fps video and the Flash player plugin discards half of them, turning the playback result into 30 fps. Now how is it that the plugin can handle this input in the first place and knows what to do with it? And why does it cripple it?
I noticed this frame drop a while ago, but what now made me even more curious is the fact that when I pause such a video and continue it after that, I get a short burst of 60 fps video. So apparently the plugin is fine with playing 60 fps video. It is deliberately told not to do it.
I also had to cripple my browser's video playing capabilities because recently Youtube started enforcing HMTL5 on Firefox, which runs like crap, so for getting the properly working Flash player back, I had to do that. But still, with all that, the Flash player plugin shows that it can interpret and play 60 fps, at least to some degree.
Then I read that - get this - the old Flash player up to version 11.5 will play 60 fps on Youtube. So now, I run Waterfox 30.0 with Flash Player 11.5 and https://www.youtube.com/html5 showing only support for HTMLVideoElement, none of the other codecs and MSE and such, and it plays 60 fps videos!
This reminds me of how my pre-Chromium Opera 12 still does certain things properly that Firefox has trouble with.
This solidifies certain suspicions that justifiedly cynical people probably had right away. Techically it is possible with very simple means, but it's not done. It is disabled intentionally. Now, I guess you could mention quality control reasons (there still might be some frame drops), but considering that Chrome is Google's browser and that Youtube is owned by Google, suspicion in this matter is healthy. Also, even with the occasional frame drop, 60 fps mode is still much better than 30 fps. It's the same as in gaming: If your game drops from 60 to 50 fps every now and then, that's no problem. And you notice such even less in video playback.
So, if you want to give it a try on your Firefox (or Opera 12 - but it drops way more frames there), too, download this:
(from this site: https://helpx.adobe.com/flash-player/kb/archived-flash-player-versions.html)
Then either also download the Flash Player uninstall tool to make the OS not block the installation of older versions, or use the MSI packages instead of the EXE installers.