It's possibly to do with a roadmap of extension APIs that are required to properly implement vertical tabs. WebExtensions Experiments allow developers to make changes to the APIs before they are in the official builds.
https://bugzilla.mozilla.org/show_bug.cgi?id=1280347
I don't know either. Might be the first release with Stylo, the new CSS engine written in Rust, but I can't really find any references to that.
Stylo is on by default for 57 at the moment.
WebRender is upcoming, but I don't think on-by-default anywhere.
(That said looks like maybe OP just mistyped.)
Does that also mean that Stylo will be the default in the release version of 57, if everything goes smoothly?
That's my understanding, yes.
Yes, I meant 57, currently the nightly. I was typing on mobile and couldn't cross check. Sorry.