For existing profiles, visit about:preferences#privacy to disable it (cf https://support.mozilla.org/en-US/kb/shield) Users just want to browse the web, not be opted-in by default to random UI experiments. See https://wiki.mozilla.org/Firefox/Shield/Shield_Studies for details. While i usually don't like diverting from upstream defaults, something that automagically loads unwanted add-ons isn't right, so i'm making an exception here.