The fact that it's in Firefox Nightly seems to blow that hypothesis out of the water
I can't really tell whether this is an official Mozilla website (though it looks like one), but the feature is certainly an official one on the roadmap. See this earlier discussion: https://news.ycombinator.com/item?id=17624709
This makes me think that it's not a feature on the roadmap of firefox sponsored by Mozilla, but an experimentation/side project.