Supposedly this will opt you out: https://support.mozilla.org/en-US/kb/telemetry-clientid https://support.mozilla.org/en-US/kb/telemetry-clientid One way is to

Supposedly this will opt you out: https://support.mozilla.org/en-US/kb/telemetry-clientid https://support.mozilla.org/en-US/kb/telemetry-clientid One way is to use your firewall to block anything going to mozilla.org or firefox.com, or the subdomains. That probably gets most of it, but possibly not all. For example, Google has a number of non-Google.com subdomains, some of which seem to be used only for telemetry. Another more involved way is to start WireShark or tcpdump and capture the traffic, then start Firefox and browse some, and then close Firefox and stop the capture. Now you have a list of all the traffic it tries to send, normal and telemetry. Sift out anything that looks suspicious and block the ip/domain via your firewall.