Actually this debate came up so often they added a special telemetry coverage extension that ignored telemetry disabled status (to great uproar) on 1% of users and reported back the number of people with telemetry disabled. As expected "more than a handful" actually turned out to be "a vocal handful" out of the 200,000,000 MAU pool.

Also those that disable telemetry after starting Firefox still sent telemetry. Mozilla only promises to delete it after 30 days not that they don't generate a number that says what % new installs in the last month disabled telemetry (though they don't publicly report this to my knowledge).

So that leaves those with whitelist only firewalling or similar measures that I (hope?) we can all agree isn't going to swing these numbers at any interesting digit position.

Yeah, first thing I do when I install Firefox is disable telemetry and install uBlock.

Is it opt out or do you choose in the installer? I forgot.

You have install, then go to the privacy settings and opt out.