Otherwise I'm not bothered. I won't be switching as long as this gets resolved within the next few days.
The cynical side of me says that it must not have this feature because if it did I'd have seen someone complaining about the browser "phoning home" or "forcing Mozilla's opinions into my eyeballs".
This isn't just a petty snipe borne out of annoyance. Ads being the malware vector that they are, and the degree of tracking and data mining out there, all of those countermeasures being turned off overnight is an exposure that should be treated with the same degree of seriousness as PII breach at a company you have an account with.
God forbid you use FoxyProxy or Tor Browser or something else that masks your connection source - this could have legitimate, real-life consequences if you don't notice the change.
On the other hand, I'm going to be honest, I have trouble reading your post without thinking things like "If you are literally trusting your life to FoxyProxy, you might want to rethink your entire internet safety strategy." Another favorite was "Defense in depth."
I've had dozens of different experiences where my extensions silently and unexpectedly malfunctioned. Configurations getting erased, new extension releases with breakage or different behaviors, maximum compatible versions in the manifest, new permissions, botched keystrokes in the extensions page, profile corruption, incompatibilities between extensions, internal bugs that cause them to crash-loop without doing anything, the works. Like, I run a ton of addons, some of which are a bit esoteric and a couple that I compile from head every few days, but even taking into account my outlier-sized surface area it's a bit silly.
I'm not going to claim that this isn't a problem. That said, blaming Mozilla for a life-threatening failure of FoxyProxy, of all things, is like blaming Cessna because a journalist flew one of their planes into a combat zone. There just wasn't any way it was going to end well.
There's only so much defending you can do against a failure like this (running Tor Browser is already pretty uncommon) and the blame for it has to be laid squarely at the feet of Mozilla for the way they chose to centralize their plugin architecture.
This is one of those low likelihood/high impact events that tend to catch everyone by surprise.. if you spend all your time as a user thinking about these failure modes (you don't.. nobody does), you'd be unable to get much else done. I'd wager the fact that the browser would suddenly gimp itself is not something the average user (even the average Tor Browser user) thinks about or plans for.