That's a rough guess; the 59 cycle just started, but it's still opt-in, not on by default. But we can say for sure it will be at least 59.

Version numbers in Nightly should be treated as works in progress, as it's built straight from the working Firefox source tree. When the current cycle ends, the code in Nightly will be bundled up to become the 59 release, and the Nightly version number will tick over to 60. Nightly displaying the 59 version number doesn't mean all features of the 59 release are present there yet.

In the nightly you need to enable it in about:config. I.e. it ships both Webrender and Gecko engine.

For the adventurous:

1) open about:config 2) set gfx.webrender.enabled to true 3) restart Nightly

For the even more adventurous:

1) open about:config 2) set gfx.webrender.enabled to true 3) set gfx.webrendest.enabled to true 4) restart Nightly