It's helpful for Mozilla's engineers to be able to measure how Firefox behaves in the real world. The Telemetry feature provides this capability by sending performance and usage information to Mozilla. As you use Firefox, Telemetry measures and collects non-personal information, such as memory consumption, responsiveness timing and feature usage. This information is then sent to Mozilla on a daily basis and used to make Firefox better for you.
Telemetry is an opt-in feature. This means that you specifically need to tell Firefox to collect and send this information to Mozilla. For more about what kind of information is collected and how it's used, see Mozilla's Firefox Browser Privacy Notice.
You can turn this on and off at any time with these steps: