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.
Somehow I remember it was supposed to be Firefox 59, not sure where I saw it now. Of course it can slip if it's not ready.
I believe the target was Firefox 59, but they didn't make it.