I don't think this is true at all. Anecdotally, a lot of Firefox users have been very receptive towards recent changes. Quantum was a huge success and I know very few people that miss the old Firefox with all of its jank.
> Mozilla need to follow suit in order to survive, rip that rendering engine out, build a new browser and a “browser toolkit” to let other people build their own
This is precisely what Servo is: https://servo.org/. Mozilla know what they're doing.