Well the limit in nightly to nvidia on windows is less about saying “we think WebRender only works/is stable on nvidia on windows” than it is about saying “we want more people to try it and possibly send big reports but we don’t want to risk breaking the entire nightly population or have them all sending in big reports, so we will start with a small subpopulation where we think bugs will be due to WebRender rather than graphics drivers”

Ah fantastic, definitely good to stagger the releases. I do understand the driver/stability situation being much more fragmented on Windows, just nice to be able to confirm how big the improvements are to Firefox that are still to come; the fastest browser by a long way IMO!

Is there any way to enable it outside nightly builds?

Yes the same instructions should work but you might get a more buggy version of Web Render as it'll be an older less debugged version.