The mentioned bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2013888
Titled: "add a prototype..."
Description:
This'll be pref-controlled and disabled by default, but will enable some fun playing around, foxfooding, and further development.
So doesn't sound like it's "landing" in the sense that your Firefox instance will actually be running this (yet), I think? Though maybe the prototyping was successful.