Supposedly this will opt you out: 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.

There is https://gist.github.com/MrYar/751e0e5f3f1430db7ec5a8c8aa237b... as well (check out the comments, too).

Or https://gist.github.com/davinian/1991bb3486cbf6005b5320e93b3... but it is quite old I think.

In any case, make sure you know what you are disabling, because in the latter it suggests disabling WebSockets which you may not want to do.

Surely there's a way of scripting this...

Something like this: https://github.com/shawnanastasio/firefox-privacy-restorer

Or you just turn it off in the normal preference UI and trust that California's AG will sue Mozilla into oblivion if they weren't honoring the CCPA.

https://blog.mozilla.org/netpolicy/2019/12/31/bringing-calif...