think the new option is actually gfx.webrender.all set to true, at least in nightly.
edit: just tested http://output.jsbin.com/surane/quiet in 61, doesn't seem to be enabled, going to try 62. Pretty sure they would have announced if it had been in 61, it's a huge addition, and it's probably coming in 62 when it's stable.
edit2: neither is it enabled in 62 (beta), also tried with the gfx.webrender.enabled option set to true just in case.
WebRender only works on Firefox Nightly regardless of what prefs you set. The code is #ifdef'd off in Beta and Release channels while there are still known bugs in Nightly.
edit: just tested http://output.jsbin.com/surane/quiet in 61, doesn't seem to be enabled, going to try 62. Pretty sure they would have announced if it had been in 61, it's a huge addition, and it's probably coming in 62 when it's stable.
edit2: neither is it enabled in 62 (beta), also tried with the gfx.webrender.enabled option set to true just in case.