about:telemetry is a new functionality included in firefox 6 and above versions.This basically is an add-on to the browser. You can install it from here
Earlier firefox team relied on some standard benchmarks such as Spidermonkey , Talos etc to test the firefox browser and optimize it accordingly. This did not seem correct as the browser was used by varied people having different system environments and configurations. Thus the Mozilla team decided to build this add-on known as about:telemetry which basically sends the browser performance statistics of every firefox user with the add-on installed to the Firefox team. This helps them analyze the varied environments the browser is being deployed and optimize on it accordingly.
This indeed seems to be great idea!!!! ALOHA
Firefox takes care to protect the privacy of the users by sending only the relevant information.
Install the add-on now , and help firefox team build better browsers optimized for your systems in the future.