A small percentage of Firefox 67 stable will have WebRender enabled. 

https://bugzilla.mozilla.org/show_bug.cgi?id=1526061

WebRender is a complete rewrite of the Gecko renderer service. Right now, browsers (including Chrome, Safari, current Firefox and possibly Edge too) does not use the GPU for the majority of webpage rendering. What they do is render most of the website on the CPU in layers, and then transfer the layers to VRAM where the GPU does compositing (and not much else). 

WebRender changes that by completely rewriting the renderer from scratch, fully intended to do as much on the GPU as possible. It's been in the works for over 3 years now and even from the get-go it showed very promising results. It's super fast, but more importantly extremely smooth. 

The problems so far have been lacking support for certain things (like CSS transitions) and some bugs, but now we've reached a point where it's ready for stable release. 

Here is a benchmark from 2016 showing chrome, Safari and Firefox struggling to get 15 fps in a Javascript benchmark. WebRender got stable 60 fps without any hiccups. 

More info here:

https://hacks.mozilla.org/2017/10/the-whole-web-at-maximum-fps-how-webrender-gets-rid-of-jank/

By the way, this is a fantastic article which explains things like draw calls, rasterization, shading and lots of other GPU components in simple and easy to understand terms. Highly recommended that people read it.