Yes, this is using WebRender by default. The WebRender 2/WebRender Next branch can render most content just fine at this point, but we went with the mainline branch for the initial nightlies, as it's more mature and stable.

Could you give a quick explanation (or link to one!) on the difference as a casual observer? Is WebRender like WebKit ?

Here's a talk I gave on it: https://air.mozilla.org/bay-area-rust-meetup-february-2016/