Hijacking the Firefox animation clock

I'm experimenting with a tool that will let the user control the animation clock used by gecko to render CSS transitions and animations, and JS animations based on requestAnimationFrame.

I'm still not sure how the UI should look like yet, and how this should interact with the recently added Paint Flashing feature. What do you think?

Proof of concept:

Code can be found here: https://github.com/paulrouget/firefox-animation-clock.