Unfortunately YouTube's 60fps support pokes a pretty big hole in the current state of Firefox.
To play back 60fps videos you need to be using the HTML5 player and stream the 1080p version. The Flash player will not work here.
The problem? Firefox doesn't support Media Source Extensions, which is what YouTube uses for DASH adaptive streaming. Mozilla's developers are working on the matter, but only for WebM for now. H.264/MP4 MSE support will have to wait.
The end result is that 1080p60 playback works great on Chrome, Safari, and even IE11, but is all but useless on Firefox.
I don't want to slag the Firefox devs too badly (hey, it's a free browser), but once again FOSS orthodoxy is getting in the way of practical feature development. H.264 support took an embarrassingly long time to come, and now Firefox is the only browser that that can't play back 1080p60 on YouTube.
Between this and their constant attempts to turn Firefox into a Chrome-alike, it's getting harder and harder to justify using Firefox.