--- a/toolkit/components/telemetry/TelemetryEnvironment.jsm
+++ b/toolkit/components/telemetry/TelemetryEnvironment.jsm
@@ -171,16 +171,17 @@ const PREF_DISTRIBUTION_VERSION = "distr
 const PREF_DISTRIBUTOR = "app.distributor";
 const PREF_DISTRIBUTOR_CHANNEL = "app.distributor.channel";
 const PREF_HOTFIX_LASTVERSION = "extensions.hotfix.lastVersion";
 const PREF_APP_PARTNER_BRANCH = "app.partner.";
 const PREF_PARTNER_ID = "mozilla.partner.id";
 const PREF_UPDATE_ENABLED = "app.update.enabled";
 const PREF_UPDATE_AUTODOWNLOAD = "app.update.auto";
 const PREF_SEARCH_COHORT = "browser.search.cohort";
+const PREF_E10S_COHORT = "e10s.rollout.cohort";
 
 const EXPERIMENTS_CHANGED_TOPIC = "experiments-changed";
 const SEARCH_ENGINE_MODIFIED_TOPIC = "browser-search-engine-modified";
 const SEARCH_SERVICE_TOPIC = "browser-search-service";
 const COMPOSITOR_CREATED_TOPIC = "compositor:created";
 const DISTRIBUTION_CUSTOMIZATION_COMPLETE_TOPIC = "distribution-customization-complete";
 
 /**
@@ -1105,16 +1106,17 @@ EnvironmentCache.prototype = {
     let updateChannel = null;
     try {
       updateChannel = UpdateUtils.getUpdateChannel(false);
     } catch (e) {}
 
     this._currentEnvironment.settings = {
       blocklistEnabled: Preferences.get(PREF_BLOCKLIST_ENABLED, true),
       e10sEnabled: Services.appinfo.browserTabsRemoteAutostart,
+      e10sCohort: Preferences.get(PREF_E10S_COHORT, "unknown"),
       telemetryEnabled: Utils.isTelemetryEnabled,
       locale: getBrowserLocale(),
       update: {
         channel: updateChannel,
         enabled: Preferences.get(PREF_UPDATE_ENABLED, true),
         autoDownload: Preferences.get(PREF_UPDATE_AUTODOWNLOAD, true),
       },
       userPrefs: this._getPrefData(),

--- a/toolkit/components/telemetry/docs/environment.rst
+++ b/toolkit/components/telemetry/docs/environment.rst
@@ -38,16 +38,17 @@ Structure::
         defaultSearchEngine: <string>, // e.g. "yahoo"
         defaultSearchEngineData: {, // data about the current default engine
           name: <string>, // engine name, e.g. "Yahoo"; or "NONE" if no default
           loadPath: <string>, // where the engine line is located; missing if no default
           submissionURL: <string> // missing if no default or for user-installed engines
         },
         searchCohort: <string>, // optional, contains an identifier for any active search A/B experiments
         e10sEnabled: <bool>, // whether e10s is on, i.e. browser tabs open by default in a different process
+        e10sCohort: <string>, // which e10s cohort was assigned for this user
         telemetryEnabled: <bool>, // false on failure
         locale: <string>, // e.g. "it", null on failure
         update: {
           channel: <string>, // e.g. "release", null on failure
           enabled: <bool>, // true on failure
           autoDownload: <bool>, // true on failure
         },
         userPrefs: {

--- a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
@@ -336,16 +336,17 @@ function checkBuildSection(data) {
     }
   }
 }
 
 function checkSettingsSection(data) {
   const EXPECTED_FIELDS_TYPES = {
     blocklistEnabled: "boolean",
     e10sEnabled: "boolean",
+    e10sCohort: "string",
     telemetryEnabled: "boolean",
     locale: "string",
     update: "object",
     userPrefs: "object",
   };
 
   Assert.ok("settings" in data, "There must be a settings section in Environment.");