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 more information, see the Firefox Data Documentation.