Well, to be fair, the last 2 Firefox "design concepts" got almost verbatim into the mainline despite almost unanimous complaints, and now I have 2 different plugins installed to correct the mess they created.
It's only a "mess" to some, I personally have no beef with Australis, and the beauty of Firefox is that it allows for you to customise it to work how you want (hence you were able to 'fix' it). This flexibility means there's still plenty of room for UI experimentation. Besides, this Lightspeed design is explicitly not for Firefox, the designers acknowledge part of what makes Firefox what it is is the customisability, this design is seeing what could be done with an alternative browser.