Some of us are responsible software owners who prefer to update on our own terms.
I understand the argument that my grandmother should probably enable auto-updates, because otherwise she could easily end up months behind on releases.
But I care deeply about my personal computing environment. I notice every minuscule change because I'm on my computer for hours and hours each day. Sometimes I'm in the middle of some important projects and I don't want anything to automatically update. Sometimes I'm really productive during an afternoon and I don't want to waste time and lose momentum on an update (or some bug, or UI change, as a result of that update). Sometimes I've heard about some problem coming down the pipe in the next update and I'd rather wait until there's mitigations to make that change work better with my specific setup.
Automatic updates basically assume that I have the computing proficiency of my grandmother. But I actually manage my computer in a very conscious, thoughtful way. All software should provide the ability to disable automatic updates (and update nagging) out of respect for power users. It's OK to hide it in a developer or advanced menu. Just give me the option.
That being said: automatic updates are a sensible default for the same reason. But let me opt out, and (Mozilla, are you listening?) for the love of god please don't override my preferences back to automatic updates when you decide to change the UI of preferences.