Update-checks are not included in telemetry. And I would think most people using addons still do update their addons from time to time, or even have the auto-check active. There is also the download-stats from their server-side, so I would think they do have a good enough picture of their numbers. Might be they could be 10% off, but surely are there not tens or even hundreds of millions of stealth-users around.

> It's probably the kind of user that has telemetry off. You don't know much if anything about them.

Don't think so, most people don't give a f** about this. Tech-people on that level are even in the industry a minority. And on the other side, those stealth-users are worthless for Mozilla, because they can't make money from google with them. So for a project needing to make money with usersnumbers, everyone who is out of this, isn't core audience anyway.