If you are interested in extending data collection by adding new probes have a look at
If you want to work with the telemetry code itself, for example to fix a bug, it is often helpful to start with these steps:
Have a look at about:telemetry to see which data is being collected and sent.
Increase the log level in about:config by setting toolkit.telemetry.log.level to Debug or Trace. This will show telemetry information in the browser console. To enable the browser console follow these instructions.
Run a local telemetry receiver, e.g. this one and set
toolkit.telemetry.serverto “localhost” (Like the next preference this needs a restart.)
toolkit.telemetry.send.overrideOfficialCheck = true, otherwise local debug builds will not send telemetry data. (Requires restart.)
More information about the internals can be found here.
Telemetry Portal - Discover all important resources for working with data
Telemetry Data Documentation - Find what data is available & how to use it