The Mozilla Firefox web browser, like the majority of browsers available today, collects Telemetry data which Mozilla introduced in Firefox 7 in 2011,
Mozilla notes on its official Mozilla Wiki site that Telemetry is "helpful for Mozilla's engineers and decision-makers" as it is used to "measure how Firefox behaves in the real world".
Telemetry provides Mozilla with "performance and usage info". Firefox collects only "non-personal information" such as "performance, hardware, usage, and customizations" according to Mozilla.
Mozilla is more open about the Telemetry that it collects. Anyone may open the Telemetry portal to check out some of the data. The Firefox Public Data Report for example provides a weekly view of activity, behavior, and hardware configurations of Firefox users.
Firefox users who want to know more about the Telemetry that Mozilla collects on their own devices can open about:telemetry to find out about that.
The data that is presented to you on the page depends on a number of factors including the Firefox channel and the Telemetry settings in Firefox.
Firefox Stable users should see a listing similar to the one on the screenshot above. The sidebar lists different data sections such as Environment Data, Session Information, or Raw JSON.
Here is a quick overview of what that data:
Currently in Nightly (additionally)
Mozilla is very open about the Telemetry that it collects and that is a good thing (Microsoft made a u-Turn in regards to Telemetry on the company's Windows 10 operating system as well). Firefox users may furthermore disable Telemetry in the browser to prevent that Telemetry data is sent to Mozilla (read here why it may not be a good idea to do so under certain circumstances).
Mozilla revealed this month that it will introduce a feature in Firefox that enables users of the browser to delete all stored Telemetry (instead of Telemetry being deleted automatically by Mozilla).
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