Telemetry is a feature that allows data collection. This is being used to collect performance metrics and other information about how Firefox performs in the wild, e.g. update events or session lengths.

There are two main ways of gathering data, Desktop Telemetry - documented here - which is used in Firefox Desktop and Glean which is Mozilla’s newer telemetry framework and used in all Mozilla projects needing data collection. Information which is gathered is called a probe in Desktop Telemetry or a metric in Glean. The data is being sent in so-called pings. When pings cannot be sent immediately, caching is implemented as well.

In many cases, Firefox on Glean (FOG) (the Firefox Desktop integration of Glean) is to be preferred over Telemetry. If your data would benefit from being in Telemetry as well as Glean, please consult the documentation for the Glean Interface For Firefox Telemetry (GIFFT).

Note: Mozilla’s data collection policy documents the process and requirements that are applied here.