I'll try to answer in the parent's place, since I've been watching this issue.
> Can you explain what you mean by "unknown Mozilla developers?" Unknown to whom?
Unknown in the sense that this extension wasn't documented at all, there was no Bugzilla issue for it and it's not clear whether it was properly vetted by QA. Whether you argue that this kind of silent push updates is good or bad, I think they aren't tested as well as in-browser functionality. This is a necessary consequence of "let's try it and revert if something breaks or people complain".
More so, a rolled back Shield study will be invisible to the users, so any problems will be impossible to debug. This is made worse by the fact that most, if not all Shield studies are opt-out, so the user won't be notified.
> Can you provide more detail on what specific configuration changes are reverted when opening the add-ons window? That sounds like a fairly serious bug.
> What is the specific "this" you're trying to "properly disable?" You shouldn't have to dive into things like lockpref.
People have reported that extensions.ui.experiment.hidden reverts after viewing the add-ons list. I haven't tried it myself, but you can find details in that Reddit thread.
Others have noticed that the Shield studies checkbox sometimes (possibly on version bumps) reverts to enabled. I can't overstate how bad this is; it's basically cheating the users' trust. Lately, Mozilla has been doing some pretty nasty things for an organization that takes pride in caring about the privacy of its users.
Are you aware of the complaints regarding Windows telemetry? Edge, for example, sends full browsing history to Microsoft by default. Should Mozilla follow suit? Because that's exactly what Pioneer does and, while it's not opt-out yet, Firefox advertises enabling it.
As for the rest of the system add-ons, they're either poorly documented (if they are at all), poorly named ("Presentation"), or seem concerning from a privacy point of view (e.g. Activity Stream, Follow-on Search Telemetry, Photon onboarding, Presentation, Web Compat Reporter).
For anyone curious, Presentation seems to be an implementation of a proposed Web API that allows browsers to find and talk to devices in their neighbourhood. Does that include location/proximity beacons like this old proposal https://hacks.mozilla.org/2013/06/the-proximity-api/ ? Do users really want Firefox to tell advertisers where they're shopping? That's the same kind of "experience improvement" that the spyware of yore used to bring.
Why should Pocket be an add-on with superpowers? There was quite a bit of backlash over it a while ago, but Mozilla didn't budge, and some employees actually spread misinformation (not to say "lied"). And actually none of my system add-ons seems to be providing any important functionality (if you disregard the new tab page, for which I haven't seen yet a privacy policy). Looking at Shield studies ( https://www.jeffersonscher.com/sumo/shield.php ), it's even worse: most are surveys, advertisements, asking the user to enable Pioneer (i.e. send full browsing history to Mozilla).