Across the different Firefox initiatives, there is a common need for a mechanism for recording, storing, sending & analysing application usage in an event-oriented format. Event Telemetry specifies a common events data format, which allows for broader, shared usage of data processing tools. Adding events is supported in artifact builds and build faster workflows.

For events recorded into Firefox Telemetry we also provide an API that opaquely handles storage and submission to our servers.


Every new or changed data collection in Firefox needs a data collection review from a Data Steward.

Serialization format

Events are submitted in an “event” ping as an array, e.g.:

  [2147, "ui", "click", "back_button"],
  [2213, "ui", "search", "search_bar", "google"],
  [2892, "ui", "completion", "search_bar", "yahoo",
    {"querylen": "7", "results": "23"}],
  [5434, "dom", "load", "frame", null,
    {"prot": "https", "src": "script"}],
  // ...

Each event is of the form:

[timestamp, category, method, object, value, extra]

Where the individual fields are:

  • timestamp: Number, positive integer. This is the time in ms when the event was recorded, relative to the main process start time.

  • category: String, identifier. The category is a group name for events and helps to avoid name conflicts.

  • method: String, identifier. This describes the type of event that occurred, e.g. click, keydown or focus.

  • object: String, identifier. This is the object the event occurred on, e.g. reload_button or urlbar.

  • value: String, optional, may be null. This is a user defined value, providing context for the event.

  • extra: Object, optional, may be null. This is an object of the form {"key": "value", ...}, both keys and values need to be strings, keys are identifiers. This is used for events where additional richer context is needed.


Each String marked as an identifier (the event name, category, method, object, and the keys of extra) is restricted to be composed of alphanumeric ASCII characters ([a-zA-Z0-9]) plus infix underscores (‘_’ characters that aren’t the first or last). category is also permitted infix periods (‘.’ characters, so long as they aren’t the first or last character).

For the Firefox Telemetry implementation, several fields are subject to length limits:

  • category: Max. byte length is 30.

  • method: Max. byte length is 20.

  • object: Max. byte length is 20.

  • value: Max. byte length is 80.

  • extra: Max. number of keys is 10.

    • Each extra key name: Max. string length is 15.

    • Each extra value: Max. byte length is 80.

Only value and the values of extra will be truncated if over the specified length. Any other String going over its limit will be reported as an error and the operation aborted.

The YAML definition file

Any event recorded into Firefox Telemetry must be registered before it can be recorded. For any code that ships as part of Firefox that happens in Events.yaml.

The probes in the definition file are represented in a fixed-depth, three-level structure. The first level contains category names (grouping multiple events together), the second level contains event names, under which the events properties are listed. E.g.:

# The following is a category of events named "browser.ui".
  click: # This is the event named "click".
    objects: ["reload-btn"] # List the objects for this event.
    description: >
      Describes this event in detail, potentially over
      multiple lines.
    # ... and more event properties.
  # ... and more events.
# This is the "dom" category.
  # And the "completion" event.
    # ...
    description: Recorded when a search completion suggestion was clicked.
      distance: The edit distance to the current search query input.
      loadtime: How long it took to load this completion entry.
  # ...

Category and event names are subject to the limits specified above.

The following event properties are valid:

  • methods (optional, list of strings): The valid event methods. If not set this defaults to [eventName].

  • objects (required, list of strings): The valid event objects.

  • description (required, string): Description of the event and its semantics.

  • release_channel_collection (optional, string): This can be set to opt-in (default) or opt-out.

  • record_in_processes (required, list of strings): A list of processes the event can be recorded in. Currently supported values are:

    • main

    • content

    • gpu

    • all_children (record in all the child processes)

    • all (record in all the processes).

  • bug_numbers (required, list of numbers): A list of Bugzilla bug numbers that are relevant to this event.

  • notification_emails (required, list of strings): A list of emails of owners for this event. This is used for contact for data reviews and potentially to email alerts.

  • expiry: There are two properties that can specify expiry, at least one needs to be set:

    • expiry_version (required, string): The version number in which the event expires, e.g. "50", or "never". A version number of type “N” is automatically converted to “N.0a1” in order to expire the event also in the development channels. For events that never expire the value never can be used.

  • extra_keys (optional, object): An object that specifies valid keys for the extra argument and a description - see the example above.

  • products (required, list of strings): A list of products the event can be recorded on. Currently supported values are:

    • firefox - Collected in Firefox Desktop for submission via Firefox Telemetry.

    • thunderbird - Collected in Thunderbird for submission via Thunderbird Telemetry.

  • operating_systems (optional, list of strings): This field restricts recording to certain operating systems only. It defaults to all. Currently supported values are:

    • mac

    • linux

    • windows

    • android

    • unix

    • all (record on all operating systems)


Combinations of category, method, and object defined in the file must be unique.


Public JS API


Services.telemetry.recordEvent(category, method, object, value, extra);

Record a registered event.

  • value: Optional, may be null. A string value, limited to 80 bytes.

  • extra: Optional. An object with string keys & values. Key strings are limited to what was registered. Value strings are limited to 80 bytes.

Throws if the combination of category, method and object is unknown. Recording an expired event will not throw, but print a warning into the browser console.


Each recordEvent of a known non-expired combination of category, method, and object, will be summarized.


Event Telemetry recording is designed to be cheap, not free. If you wish to record events in a performance-sensitive piece of code, store the events locally and record them only after the performance-sensitive piece (“hot path”) has completed.


Services.telemetry.recordEvent("ui", "click", "reload-btn");
// event: [543345, "ui", "click", "reload-btn"]
Services.telemetry.recordEvent("ui", "search", "search-bar", "google");
// event: [89438, "ui", "search", "search-bar", "google"]
Services.telemetry.recordEvent("ui", "completion", "search-bar", "yahoo",
                               {"querylen": "7", "results": "23"});
// event: [982134, "ui", "completion", "search-bar", "yahoo",
//           {"qerylen": "7", "results": "23"}]


Services.telemetry.setEventRecordingEnabled(category, enabled);

Event recording is currently disabled by default for events registered in Events.yaml. Dynamically-registered events (those registered using registerEvents()) are enabled by default, and cannot be disabled. Privileged add-ons and Firefox code can enable & disable recording events for specific categories using this function.


Services.telemetry.setEventRecordingEnabled("ui", true);
// ... now events in the "ui" category will be recorded.
Services.telemetry.setEventRecordingEnabled("ui", false);
// ... now "ui" events will not be recorded anymore.


Even if your event category isn’t enabled, counts of events that attempted to be recorded will be summarized.


Services.telemetry.registerEvents(category, eventData);

Register new events from add-ons.

  • category - (required, string) The category the events are in.

  • eventData - (required, object) An object of the form {eventName1: event1Data, ...}, where each events data is an object with the entries:

    • methods - (required, list of strings) The valid event methods.

    • objects - (required, list of strings) The valid event objects.

    • extra_keys - (optional, list of strings) The valid extra keys for the event.

    • record_on_release - (optional, bool)

    • expired - (optional, bool) Whether this event entry is expired. This allows recording it without error, but it will be discarded. Defaults to false.

For events recorded from add-ons, registration happens at runtime. Any new events must first be registered through this function before they can be recorded. The registered categories will automatically be enabled for recording, and cannot be disabled. If a dynamic event uses the same category as a static event, the category will also be enabled upon registration.

After registration, the events can be recorded through the recordEvent() function. They will be submitted in event pings like static events are, under the dynamic process.

New events registered here are subject to the same limitations as the ones registered through Events.yaml, although the naming was in parts updated to recent policy changes.

When add-ons are updated, they may re-register all of their events. In that case, any changes to events that are already registered are ignored. The only exception is expiry; an event that is re-registered with expired: true will not be recorded anymore.


Services.telemetry.registerEvents("myAddon.interaction", {
  "click": {
    methods: ["click"],
    objects: ["red_button", "blue_button"],
// Now events can be recorded.
Services.telemetry.recordEvent("myAddon.interaction", "click", "red_button");

Internal API

Services.telemetry.snapshotEvents(dataset, clear, eventLimit);

These functions are only supposed to be used by Telemetry internally or in tests.

Also, the event-telemetry-storage-limit-reached topic is notified when the event ping event limit is reached (1000 event records). This is intended only for use internally or in tests.

Event Summary

Calling recordEvent on any non-expired registered event will accumulate to a Scalar for ease of analysing uptake and usage patterns. Even if the event category isn’t enabled.

The scalar is telemetry.event_counts for statically-registered events (the ones in Events.yaml) and telemetry.dynamic_event_counts for dynamically-registered events (the ones registered via registerEvents). These are keyed scalars where the keys are of the form category#method#object and the values are counts of the number of times recordEvent was called with that combination of category, method, and object.

These two scalars have a default maximum key limit of 500 per process.


// telemetry.event_counts summarizes in the same process the events were recorded

// Let us suppose in the parent process this happens:
Services.telemetry.recordEvent("interaction", "click", "document", "xuldoc");
Services.telemetry.recordEvent("interaction", "click", "document", "xuldoc-neighbour");

// And in each of child processes 1 through 4, this happens:
Services.telemetry.recordEvent("interaction", "click", "document", "htmldoc");

In the case that is statically-registered, this will result in the parent-process scalar telemetry.event_counts having a key interaction#click#document with value 2 and the content-process scalar telemetry.event_counts having a key interaction#click#document with the value 4.

All dynamically-registered events end up in the dynamic-process telemetry.dynamic_event_counts (notice the different name) regardless of in which process the events were recorded. From the example above, if was registered with registerEvents then the dynamic-process scalar telemetry.dynamic_event_counts would have a key interaction#click#document with the value 6.


Tests involving Event Telemetry often follow this four-step form:

  1. Services.telemetry.clearEvents(); To minimize the effects of prior code and tests.

  2. Services.telemetry.setEventRecordingEnabled(myCategory, true); To enable the collection of your events. (May or may not be relevant in your case)

  3. runTheCode(); This is part of the test where you call the code that’s supposed to collect Event Telemetry.

  4. TelemetryTestUtils.assertEvents(expected, filter, options); This will check the events recorded by Event Telemetry against your provided list of expected events. If you only need to check the number of events recorded, you can use TelemetryTestUtils.assertNumberOfEvents(expectedNum, filter, options);. Both utilities have helpful inline documentation.

Version History

  • Firefox 79: geckoview support removed (see bug 1620395).

  • Firefox 52: Initial event support (bug 1302663).

  • Firefox 53: Event recording disabled by default (bug 1329139).

  • Firefox 54: Added child process events (bug 1313326).

  • Firefox 56: Added support for recording new probes from add-ons (bug 1302681).

  • Firefox 58:

    • Ignore re-registering existing events for a category instead of failing (bug 1408975).

    • Removed support for the expiry_date property, as it was unused (bug 1414638).

  • Firefox 61:

    • Enabled support for adding events in artifact builds and build-faster workflows (bug 1448945).

    • Added summarization of events (bug 1440673).

  • Firefox 66: Replace cpp_guard with operating_systems (bug 1482912)`