Mozilla literally cannot win here; no matter what they do people will just get angrier. They should just strip out the whole Firefox UI and make you run it from a terminal.
Sure, there are some people who make very minimal usage of a web browser. They can get by just fine with a dumbed down Chrome-style UI. They might even like that experience.
Not everybody falls into that camp, however. There are a lot of users who push web browsers much harder, and expect a different experience. They need a UI that exposes the functionality they need to constantly use.
Firefox used to be good at giving power users what they needed, while still being relatively lean, easy-to-use and useful for users who pushed it less intensively.
We've seen this change over the past few years, however, as they've stripped out useful UI functionality in their quest to imitate Chrome. Firefox is now a weaker, less-practical browser for many people. These people are burdened with trying to install numerous addons, just to restore Firefox to something reasonably close to its previous state of usability.
Firefox isn't alone, of course. Just look at the whole debacle that Opera has experienced with the new versions of their desktop browser. It's perhaps even worse in that case, given how they're building upon Blink and Chromium directly. The new versions of Opera are horrible compared to Opera 12 and earlier. It's just really unfortunate to see this happening to software products that were once pretty good.
I mostly like Australis. And I can see why Mozilla doesn't listen to most of the noise about it. As with everything else, nobody gets really loud about how much better it is.
The fact that we overwhelmingly hear silence or anger toward Mozilla's UI changes is indicative of them not being very good at all.