How do I stop Mozilla telemetry?

If you go to Tools -> Options -> Advanced -> Data Choices you can untick the the options there.

Or... go to about:config in Firefox and search for "telemetry" and set toolkit.telemetry.server to 0.0.0.0, or some other non-existent IP. Also be sure to disable toolkit.telemetry.unified, toolkit.telemetry.enabled and toolkit.telemetry.archive.enabled. When I say disable a setting, I mean set it to False.

Enjoy assurance you're not being spied on by companies! Except if you use Windows 10...