Have been playing around with Firefox with the aim of finding out whether I can remove the telemetry entirely. The answer is not easily. It would be a more involved project.

Firefox sends telemetry to 52.88.27.118 unencrypted so that third parties will also be able to see what's going on. Since this is opt out it must be a major data exfiltration route.

I've blocked that IP address within #freedombone just to be on the safe side, although it's not expected that anyone will have Firefox installed on a server.