It's helpful for Mozilla's engineers and decision-makers to be able to measure how Firefox behaves in the real world. The Telemetry feature provides this capability by sending performance and usage info to Mozilla. As you use Firefox, Telemetry measures and collects non-personal information, such as performance, hardware, usage and customizations. It then sends this information to Mozilla on a daily basis and we use it to improve Firefox.
For detailed technical information on how this data is collected, as well as information on how to add Telemetry to Firefox, see Telemetry reference in the Firefox source documentation.
For more information on accessing and using this information in analysis, see the Firefox Data Documentation.