The T in DNT stands for Do Not Track, not Do Not Telemetry. Telemetry and Tracking are not mutually exclusive; both can be implemented by software developers.

The T in DNT stands for Do Not Track, not Do Not Telemetry. Telemetry and Tracking are not mutually exclusive; both can be implemented by software developers. Telemetry is unidentified software usage statistics from a random IP address somewhere on the Internet, generally without caring about whose telemetry it is. Tracking is an attempt to identify who is operating the software, in order to associate that software's activity with specific human beings in a database. DNT is Do Not Track, which means "do not attempt to identify me" — but I assume that Firefox telemetry typically isn't attempting to identify you in the first place, so making a statement about tracking with the DNT checkbox would not affect telemetry decisions.