I actually was the one who convinced the FireFox people to support 120Hz monitors -- see FireFox Bugzilla #856427: Add vsync support to Windows. You can see from my comments in the bugs that I eventually convinced them to allow animations to run at 120fps on 120Hz monitors. They are now working on other bugs that is causing the juddering/stuttering, they are already familiar with it. So FireFox will do the animation as good as Chrome, eventually.
Meanwhile, Opera 15 also runs the animations pretty well too at 120fps. IE10 works great but has a bug that limits VSYNC animations to 105Hz, I've reported the bug to Microsoft already in their bug tracking system (and acknowledged by Microsoft). But of course, they're a lot harder to reach than the Chrome/FF teams. And for example, there are situations where I can run at 120fps but still gets lots of stutters in FireFox. This pattern this stutters a lot in FireFox in full screen mode on 120Hz monitors (not necessarily on 60Hz) because it can't consume more than about ~2% of CPU before it begins to stutter.
It is a challenge to make all web browsers run perfectly at fps=Hz but this recently became possible in 2012 onwards, with the advent of GPU-accelerated web browsers that is gaining the ability to run animations synchronized to VSYNC in pure HTML5 with zero plugins. (TestUFO does not use Flash! It even works on iPad which even also synchronized to VSYNC too.)
For now I need to give people the boilerplate advice to use Chrome, because just about any version of Chrome (not running multimonitor) will usually run this test reliably. Opera 15 now uses the Webkit engine (Chrome based) and manages to sync to VSYNC now too. Rest assured, I'm coaxing FireFox and Microsoft to fix their respective issues, so I can also recommend them too, so that all four major browsers all have the capability to animate to VSYNC at 120fps @ 120Hz. I am calling out an existing new 2012 W3C standard draft that recommends animations running to refresh rate, just that not all browser makers have yet have had a widespread test tool as good as TestUFO, to test their web browsers with, and also to obtain a 120Hz monitor (Mozilla even purhcased a 120Hz monitor based on my advice in Bug #856427).
Edited by Mark Rejhon - 7/21/13 at 8:25pm