A brief history of Mozilla data collection

This section was originally included in the Project Smoot existing metrics report (Mozilla internal link); the DTMO version has been updated to reflect changes to the data platform.

blocklist.xml and Active Daily Installs (ADI)

The blocklist was a mechanism for informing Firefox clients about malicious add-ons, DLLs, and other extension content that should be blocked. The blocklist also noted when hardware acceleration features should be avoided on certain graphics cards. To be effective, the blocklist needed to be updated on a faster cadence than Firefox releases.

The blocklist was first implemented in 2006 for Firefox 2, and reported the app ID and version to the blocklist server.

Several additional variables, including OS version, locale, and distribution, were added to the URL for Firefox 3 in 2008. Being able to count users was already expressed as a priority in the bug comments.

A count of blocklist fetches was used to produce a metric called Active Daily Users, which was renamed to Active Daily Installs (ADI) by 2012.

As of August 2020, this mechanism has been superseded by a Remote Settings-based replacement and the ADI measure is no longer in use. See the historical reference on ADI for more information.


The earliest telemetry infrastructure landed in Firefox 6, and was driven by engineering needs.

Telemetry was originally opt-out on the nightly and aurora channels, and opt-in otherwise. It originally lacked persistent client identifiers.

Firefox Health Report

The Firefox Health Report (FHR) was specified to enable longitudinal and retention analyses. FHR aimed to enable analyses that were not possible based on the blocklist ping, update ping, telemetry, Test Pilot and crash stats datasets that were already available.

FHR was first implemented in Firefox 20. It was introduced in blog posts by Mitchell Baker and Gilbert Fitzgerald.

To avoid introducing a persistent client identifier, FHR originally relied on a “document ID” system. The client would generate a new UUID (a random, unique ID) for each FHR document, and remember a list of its most recent previous document IDs. While uploading a new FHR document, the client would ask the server to remove its previous documents. The intent was that the server would end up holding at most one document from each user, and longitudinal metrics could be accumulated by the client. This approach proved fragile and was abandoned. A persistent client identifier was implemented for Firefox 30.

Firefox Desktop Telemetry today

FHR was retired and merged with telemetry to produce the current generation of telemetry data, distinguished as “v4 telemetry” or “unified telemetry.”

Instead of mapping FHR probes directly to telemetry, the Unified Telemetry project built upon the telemetry system to answer the questions Mozilla had attempted to answer with FHR.

The implementation of unified telemetry and opt-out delivery to the release channel was completed for Firefox 42, in 2015.

Telemetry payloads are uploaded in documents called pings. Several kinds of pings are defined, representing different kinds of measurement. These include:

  • main: activity, performance, technical, and other measurements; the workhorse of Firefox desktop telemetry
  • crash: information about crashes, including stack traces
  • opt-out: a farewell ping sent when a user disables telemetry
  • module: on Windows, records DLLs injected into the Firefox process

and others.

Browser sessions and subsessions are important concepts in telemetry. A session begins when the browser launches and ends—perhaps seconds or days later— when the parent browser process terminates.

A subsession ends

  • when its parent session ends, or
  • at local midnight, or
  • when the telemetry environment changes,

whichever comes first.

The telemetry environment describes the hardware and operating system of the client computer. It can change during a Firefox session when e.g. hardware is plugged into a laptop.

The subsession is the reporting unit for activity telemetry; each main ping describes a single subsession. Activity counters are reset once a subsession ends. Data can be accumulated for analysis by summing over a client’s pings.

Telemetry pings can contain several different types of measurements:

  • scalars are integers describing either an event count or a measurement that occurs only once during a subsession; simpleMeasurements are an older, less flexible scalar implementation in the process of being deprecated
  • histograms represent measurements that can occur repeatedly during a subsession; histograms report the count of measurements that fell into each of a set of predefined buckets (e.g. between zero and one, between one and two, etc).
  • events represent discrete events; the time and ordering of the events are preserved, which clarifies sequences of user actions

Data types are discussed in more depth in the telemetry data collection documentation.

Firefox Desktop Telemetry: The Next Generation

The next step for Firefox Desktop Telemetry is to prototype an implementation using Glean.

This effort is known as "Firefox on Glean" or FOG. This effort is expected to begin in late 2019 / early 2020.