Bad news is that it is not clear at this point whether Mozilla is going to go forward with the implementation. A post on Reddit by one of the project members suggests that the build is a "rough proof-of-concept". Some features tested in the build "did not survive". It is unclear which did not, as they are not mentioned. Mozilla is, however, implementing those that survived the cut into Firefox. Again, the poster does not mention which those are. It is also not verified that the poster is actually a member of the project team, so take this with a grain of salt as well.
Isn't Firefox open source? So isn't it possible that anyone could see the changes being made even in the nightly versions? I'm not a programmer so forgive my ignorance.
UX is a very subjective matter.
Yo fuck it. This comment saved my life.
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More like: Looks Cool
Versus how it is now: Looks cool
or if you have a shit ton of tabs open: lo... co...
What you are doing does not at all, besides not having to do anything with the topic at hand.