If I understod correctly (which isn’t certain) Skia is also the rendering backend for chromium. Chromium is used as the rendering bits for electron. Electron is how you make these apps on “web engines”. So all those editors based on web engines likely render through Skia...
WebRender is not even enabled by default in Nightly yet. It’ll still be a while before it’s ready for production—definitely not Firefox 59. I don’t know of any schedule, but I haven’t been following it much either.
But yes, the Quantum Render project is all about making WebRender Firefox’s primary graphics backend.