I was skeptical of the move to Firefox Quantum originally because of the extension-breaking, but after it happened, I can see the motivation: after upgrading, Firefox got a lot faster and less prone to memory leaks for me, at least on macOS. As I understand it, the performance improvements in the new Quantum engine were incompatible with the way the old extension API worked, so a new one needed to be introduced. To me the tradeoff was worth it, because the increasing sluggishness of FF compared to Chrome, pre-Quantum, was making it harder to stick with.
I don't think anyone denies the performance improvements and other advantages of the Quantum project. The claim is that the transition wasn't handled well, as replacement APIs for the deprecated addon system weren't in place until several releases after Quantum debuted, and several of the more deep-reaching abilities addons had were deprecated completely. This broke many people's workflows, destroying trust.
This also happened a short time after Mozilla previously broke addons with the e10s changes, requiring many (most?) of them to be rewritten.
Basically, people don't trust Mozilla to handle transitions well any more, even when they happen to be worthwhile transitions.
This was my exact experience as well. I had to kill/restart FF every few days because it would randomly start eating CPU (this was on Win 10). Because of this I used Chrome until they disabled back navigation via backspace and forced alt-left instead. This pissed me off enough to try FF again, which was right around when Quantum came out. I've never looked back.
