It seems now it’s ready for some testing
https://groups.google.com/forum/#!topic/mozilla.dev.platform/x_sFbDgQJP0
In bug 1490742 I have enabled WebRender in Nightly on non-laptop
Windows 10 Nvidia (~17% of our Nightly audience). This is a rewrite of
much the graphics backend in Firefox. We expect some edge-case
regressions, but generally nothing serious. We have quite a few staff
and volunteers who have been using WebRender for months without major
issues.If you’re on this hardware and you see a problem please file a bug.
You can check if you’re using WebRender by looking at the Compositing
section of about:support. Further, WebRender should be generally
usable on all platforms other than Android right now so if you want to
be keen you can try it out now with the gfx.webrender.all pref.