> you also need to explicitly opt-in
Wrong, as far as I see: Looking in my about:config, I see
app.shield.optoutstudies.enabled=true
browser.onboarding.shieldstudy.enabled=true
enabled by default. The settings that I've changed from the default are shown in bold. These aren't bold. Those are the defaults. Everybody can check.
That means that the user must actively take steps to disable them, if he knows that they exist and where he can disable them.
Every time the user creates a new profile, and most probably also when he "refreshes" an old one, he has by default the studies allowed.
It's even worse in other aspects: through the UI the "Allow Firefox to install and run studies" can be unchecked but it doesn't change the value of "experiments.enabled" to false in about:config.
Apparently the "experiments" allow Mozilla to install the "experimental" extensions to any user, without him knowing. And these extensions are invisible in the GUI! Even if the user goes to the about:config and sets extensions.ui.experiment.hidden to false, it will be automatically set to true again.