--- a/toolkit/components/telemetry/app/TelemetryEnvironment.jsm
+++ b/toolkit/components/telemetry/app/TelemetryEnvironment.jsm
@@ -53,16 +53,17 @@ ChromeUtils.defineModuleGetter(
 // The maximum length of a string (e.g. description) in the addons section.
 const MAX_ADDON_STRING_LENGTH = 100;
 // The maximum length of a string value in the settings.attribution object.
 const MAX_ATTRIBUTION_STRING_LENGTH = 100;
 // The maximum lengths for the experiment id and branch in the experiments section.
 const MAX_EXPERIMENT_ID_LENGTH = 100;
 const MAX_EXPERIMENT_BRANCH_LENGTH = 100;
 const MAX_EXPERIMENT_TYPE_LENGTH = 20;
+const MAX_EXPERIMENT_ENROLLMENT_ID_LENGTH = 40;
 
 /**
  * This is a policy object used to override behavior for testing.
  */
 // eslint-disable-next-line no-unused-vars
 var Policy = {
   now: () => new Date(),
   _intlLoaded: false,
@@ -120,16 +121,17 @@ var TelemetryEnvironment = {
    * Add an experiment annotation to the environment.
    * If an annotation with the same id already exists, it will be overwritten.
    * This triggers a new subsession, subject to throttling.
    *
    * @param {String} id The id of the active experiment.
    * @param {String} branch The experiment branch.
    * @param {Object} [options] Optional object with options.
    * @param {String} [options.type=false] The specific experiment type.
+   * @param {String} [options.enrollmentId=undefined] The id of the enrollment.
    */
   setExperimentActive(id, branch, options = {}) {
     if (gGlobalEnvironment) {
       gGlobalEnvironment.setExperimentActive(id, branch, options);
     } else {
       gActiveExperimentStartupBuffer.set(id, { branch, options });
     }
   },
@@ -1172,23 +1174,40 @@ EnvironmentCache.prototype = {
       if (type.length != options.type.length) {
         options.type = type;
         this._log.warn(
           "setExperimentActive - the experiment type was truncated."
         );
       }
     }
 
+    // Truncate the enrollment id if present.
+    if (options.hasOwnProperty("enrollmentId")) {
+      let enrollmentId = limitStringToLength(
+        options.enrollmentId,
+        MAX_EXPERIMENT_ENROLLMENT_ID_LENGTH
+      );
+      if (enrollmentId.length != options.enrollmentId.length) {
+        options.enrollmentId = enrollmentId;
+        this._log.warn(
+          "setExperimentActive - the enrollment id was truncated."
+        );
+      }
+    }
+
     let oldEnvironment = Cu.cloneInto(this._currentEnvironment, myScope);
     // Add the experiment annotation.
     let experiments = this._currentEnvironment.experiments || {};
     experiments[saneId] = { branch: saneBranch };
     if (options.hasOwnProperty("type")) {
       experiments[saneId].type = options.type;
     }
+    if (options.hasOwnProperty("enrollmentId")) {
+      experiments[saneId].enrollmentId = options.enrollmentId;
+    }
     this._currentEnvironment.experiments = experiments;
     // Notify of the change.
     this._onEnvironmentChange("experiment-annotation-changed", oldEnvironment);
   },
 
   setExperimentInactive(id) {
     this._log.trace("setExperimentInactive");
     let experiments = this._currentEnvironment.experiments || {};

--- a/toolkit/components/telemetry/docs/collection/experiments.rst
+++ b/toolkit/components/telemetry/docs/collection/experiments.rst
@@ -5,17 +5,17 @@ This API allows privileged JavaScript to
 
 The experiment annotations are sent with any ping that includes the :doc:`../data/environment` data.
 
 The JS API
 ==========
 Privileged JavaScript code can annotate experiments using the functions exposed by ``TelemetryEnvironment.jsm``.
 
 The following function adds an annotation to the environment for the provided ``id``, ``branch`` and ``options``. Calling this function repeatedly with the same ``id`` will overwrite the state and trigger new subsessions (subject to throttling).
-``options`` is an object that currently may contain ``type``, to tag the experiment with a specific type.
+``options`` is an object that may contain ``type`` to tag the experiment with a specific type or ``enrollmentId`` to tag the enrollment in this experiment with an identifier.
 
 .. code-block:: js
 
     TelemetryEnvironment.setExperimentActive(id, branch, [options={}}])
 
 This removes the annotation for the experiment with the provided ``id``.
 
 .. code-block:: js
@@ -30,11 +30,12 @@ This synchronously returns a dictionary 
 
 .. note::
 
     Both ``setExperimentActive`` and ``setExperimentInactive`` trigger a new subsession. However
     the latter only does so if there was an active experiment with the provided ``id``.
 
 Limits and restrictions
 -----------------------
-To prevent abuses, the content of both the experiment ``id`` and ``branch`` is limited to
+To prevent abuses, the content of the experiment ``id`` and ``branch`` is limited to
 100 characters in length.
 ``type`` is limited to a length of 20 characters.
+``enrollmentId`` is limited to 40 characters (chosen to be just a little longer than the 36-character long GUID text representation).

--- a/toolkit/components/telemetry/docs/data/environment.rst
+++ b/toolkit/components/telemetry/docs/data/environment.rst
@@ -279,17 +279,17 @@ Structure:
                 version: <string>,
                 userDisabled: <bool>,
                 applyBackgroundUpdates: <integer>,
             },
             ...
         },
       },
       experiments: {
-        "<experiment id>": { branch: "<branch>" },
+        "<experiment id>": { branch: "<branch>", type: "<type>", enrollmentId: "<id>" },
         // ...
       }
     }
 
 build
 -----
 
 buildId
@@ -460,22 +460,32 @@ Just like activeAddons, up-to-date infor
 
 activeGMPPlugins
 ~~~~~~~~~~~~~~~~
 
 Just like activePlugins, this will report dummy values until the blocklist is loaded.
 
 experiments
 -----------
-For each experiment we collect the ``id`` and the ``branch`` the client is enrolled in. Both fields are truncated to 100 characters and a warning is printed when that happens.
+For each experiment we collect the
 
+- ``id`` (Like ``hotfix-reset-xpi-verification-timestamp-1548973``, max length 100 characters)
+- ``branch`` (Like ``control``, max length 100 characters)
+- ``type`` (Optional. Like ``normandy-exp``, max length 20 characters)
+- ``enrollmentId`` (Optional. Like ``5bae2134-e121-46c2-aa00-232f3f5855c5``, max length 40 characters)
+
+In the event any of these fields are truncated, a warning is printed to the console.
 
 Version History
 ---------------
 
+- Firefox 70:
+
+  - Added ``experiments.<experiment id>.enrollmentId``. (`bug 1555172 <https://bugzilla.mozilla.org/show_bug.cgi?id=1555172>`_)
+
 - Firefox 67:
 
-  - Removed ``persona``. The ``addons.activeAddons`` list should be used instead. (`bug 1525511 https://bugzilla.mozilla.org/show_bug.cgi?id=1525511>`_)
+  - Removed ``persona``. The ``addons.activeAddons`` list should be used instead. (`bug 1525511 <https://bugzilla.mozilla.org/show_bug.cgi?id=1525511>`_)
 
 - Firefox 61:
 
   - Removed empty ``addons.activeExperiment`` (`bug 1452935 <https://bugzilla.mozilla.org/show_bug.cgi?id=1452935>`_).