That is odd because the three.js demos i run report well in excess of 60 fps in FF when my monitor refresh rate is set 144Hz. The stats.js component only measures up to 100FPS and the simple demos often exceed that.

edit: I also have G-SYNC enabled and I noticed that the bug thread says the FF syncs to vsync on Windows. nVidia's G-SYNC might be messing with vsync to achieve higher than 60FPS on supported monitors?