diff --git a/toolkit/components/telemetry/Histograms.json b/toolkit/components/telemetry/Histograms.json
index 9c166fbd5e4..5d23214d4b8 100644
--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -3320,6 +3320,34 @@
     "n_buckets": "1000",
     "description": "The time (in milliseconds) that it took a 'detach' request to go round trip."
   },
+  "COOKIES_3RDPARTY_NUM_SITES_ACCEPTED": {
+    "kind": "linear",
+    "low": "5",
+    "high": "145",
+    "n_buckets": "30",
+    "description": "The number of distinct pairs (first-party site, third-party site attempting to set cookie) for which the third-party cookie has been accepted. Sites are considered identical if they have the same eTLD + 1. Measures are normalized per 24h."
+  },
+  "COOKIES_3RDPARTY_NUM_SITES_BLOCKED": {
+    "kind": "linear",
+    "low": "5",
+    "high": "145",
+    "n_buckets": "30",
+    "description": "The number of distinct pairs (first-party site, third-party site attempting to set cookie) for which the third-party cookie has been rejected. Sites are considered identical if they have the same eTLD + 1. Measures are normalized per 24h."
+  },
+  "COOKIES_3RDPARTY_NUM_ATTEMPTS_ACCEPTED": {
+    "kind": "linear",
+    "low": "10",
+    "high": "500",
+    "n_buckets": "50",
+    "description": "The total number of distinct attempts by third-party sites to place cookies which have been accepted.  Measures are normalized per 24h."
+  },
+  "COOKIES_3RDPARTY_NUM_ATTEMPTS_BLOCKED": {
+    "kind": "linear",
+    "low": "10",
+    "high": "500",
+    "n_buckets": "50",
+    "description": "The total number of distinct attempts by third-party sites to place cookies which have been rejected.  Measures are normalized per 24h."
+  },
   "DEVTOOLS_DEBUGGER_RDP_LOCAL_BLACKBOX_MS": {
     "kind": "exponential",
     "high": "10000",
diff --git a/toolkit/components/telemetry/Makefile.in b/toolkit/components/telemetry/Makefile.in
index c7a48854bcd..b790e9b5dd1 100644
--- a/toolkit/components/telemetry/Makefile.in
+++ b/toolkit/components/telemetry/Makefile.in
@@ -28,6 +28,7 @@ DISABLED_EXTRA_COMPONENTS = \
 
 EXTRA_JS_MODULES = \
   TelemetryStopwatch.jsm \
+  ThirdPartyCookieProbe.jsm \
   $(NULL)
 
 LOCAL_INCLUDES += -I$(topsrcdir)/xpcom/build
diff --git a/toolkit/components/telemetry/TelemetryPing.js b/toolkit/components/telemetry/TelemetryPing.js
index 2cd116b029a..5c6c39b2bbd 100644
--- a/toolkit/components/telemetry/TelemetryPing.js
+++ b/toolkit/components/telemetry/TelemetryPing.js
@@ -15,6 +15,7 @@ Cu.import("resource://gre/modules/NetUtil.jsm");
 Cu.import("resource://gre/modules/LightweightThemeManager.jsm");
 #endif
 Cu.import("resource://gre/modules/ctypes.jsm");
+Cu.import("resource://gre/modules/ThirdPartyCookieProbe.jsm");
 
 // When modifying the payload in incompatible ways, please bump this version number
 const PAYLOAD_VERSION = 1;
@@ -761,6 +762,10 @@ TelemetryPing.prototype = {
    * Initializes telemetry within a timer. If there is no PREF_SERVER set, don't turn on telemetry.
    */
   setup: function setup() {
+    // Initialize some probes that are kept in their own modules
+    this._thirdPartyCookies = new ThirdPartyCookieProbe();
+    this._thirdPartyCookies.init();
+
     // Record old value and update build ID preference if this is the first
     // run with a new build ID.
     let previousBuildID = undefined;
@@ -1002,7 +1007,7 @@ TelemetryPing.prototype = {
    * Remove observers to avoid leaks
    */
   uninstall: function uninstall() {
-    this.detachObservers()
+    this.detachObservers();
     if (this._hasWindowRestoredObserver) {
       Services.obs.removeObserver(this, "sessionstore-windows-restored");
       this._hasWindowRestoredObserver = false;
diff --git a/toolkit/components/telemetry/ThirdPartyCookieProbe.jsm b/toolkit/components/telemetry/ThirdPartyCookieProbe.jsm
new file mode 100644
index 00000000000..ffb87664994
--- /dev/null
+++ b/toolkit/components/telemetry/ThirdPartyCookieProbe.jsm
@@ -0,0 +1,180 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+let Ci = Components.interfaces;
+let Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+
+this.EXPORTED_SYMBOLS = ["ThirdPartyCookieProbe"];
+
+const MILLISECONDS_PER_DAY = 1000 * 60 * 60 * 24;
+
+/**
+ * A probe implementing the measurements detailed at
+ * https://wiki.mozilla.org/SecurityEngineering/ThirdPartyCookies/Telemetry
+ *
+ * This implementation uses only in-memory data.
+ */
+this.ThirdPartyCookieProbe = function() {
+  /**
+   * A set of third-party sites that have caused cookies to be
+   * rejected. These sites are trimmed down to ETLD + 1
+   * (i.e. "x.y.com" and "z.y.com" are both trimmed down to "y.com",
+   * "x.y.co.uk" is trimmed down to "y.co.uk").
+   *
+   * Used to answer the following question: "For each third-party
+   * site, how many other first parties embed them and result in
+   * cookie traffic?" (see
+   * https://wiki.mozilla.org/SecurityEngineering/ThirdPartyCookies/Telemetry#Breadth
+   * )
+   *
+   * @type Map<string, RejectStats> A mapping from third-party site
+   * to rejection statistics.
+   */
+  this._thirdPartyCookies = new Map();
+  /**
+   * Timestamp of the latest call to flush() in milliseconds since the Epoch.
+   */
+  this._latestFlush = Date.now();
+};
+
+this.ThirdPartyCookieProbe.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+  init: function() {
+    Services.obs.addObserver(this, "profile-before-change", false);
+    Services.obs.addObserver(this, "third-party-cookie-accepted", false);
+    Services.obs.addObserver(this, "third-party-cookie-rejected", false);
+  },
+  dispose: function() {
+    Services.obs.removeObserver(this, "profile-before-change");
+    Services.obs.removeObserver(this, "third-party-cookie-accepted");
+    Services.obs.removeObserver(this, "third-party-cookie-rejected");
+  },
+  /**
+   * Observe either
+   * - "profile-before-change" (no meaningful subject or data) - time to flush statistics and unregister; or
+   * - "third-party-cookie-accepted"/"third-party-cookie-rejected" with
+   *    subject: the nsIURI of the third-party that attempted to set the cookie;
+   *    data: a string holding the uri of the page seen by the user.
+   */
+  observe: function(docURI, topic, referrer) {
+    try {
+      if (topic == "profile-before-change") {
+        // A final flush, then unregister
+        this.flush();
+        this.dispose();
+      }
+      if (topic != "third-party-cookie-accepted"
+          && topic != "third-party-cookie-rejected") {
+        // Not a third-party cookie
+        return;
+      }
+      // Add host to this._thirdPartyCookies
+      let firstParty = normalizeHost(referrer);
+      let thirdParty = normalizeHost(docURI.QueryInterface(Ci.nsIURI).host);
+      let data = this._thirdPartyCookies.get(thirdParty);
+      if (!data) {
+        data = new RejectStats();
+        this._thirdPartyCookies.set(thirdParty, data);
+      }
+      if (topic == "third-party-cookie-accepted") {
+        data.addAccepted(firstParty);
+      } else {
+        data.addRejected(firstParty);
+      }
+    } catch (ex) {
+      // Errors should not remain silent
+      Services.console.logStringMessage("ThirdPartyCookieProbe: Uncaught error " + ex + "\n" + ex.stack);
+    }
+  },
+
+  /**
+   * Clear internal data, fill up corresponding histograms.
+   */
+  flush: function(aUptime) {
+    let now = Date.now();
+    let updays = (now - this._latestFlush) / MILLISECONDS_PER_DAY;
+    if (updays <= 0) {
+      // Unlikely, but regardless, don't risk division by zero
+      // or weird stuff.
+      return;
+    }
+    this._latestFlush = now;
+    let acceptedSites = Services.telemetry.getHistogramById("COOKIES_3RDPARTY_NUM_SITES_ACCEPTED");
+    let rejectedSites = Services.telemetry.getHistogramById("COOKIES_3RDPARTY_NUM_SITES_BLOCKED");
+    let acceptedRequests = Services.telemetry.getHistogramById("COOKIES_3RDPARTY_NUM_ATTEMPTS_ACCEPTED");
+    let rejectedRequests = Services.telemetry.getHistogramById("COOKIES_3RDPARTY_NUM_ATTEMPTS_BLOCKED");
+    for (let [k, data] of this._thirdPartyCookies) {
+      acceptedSites.add(data.countAcceptedSites / updays);
+      rejectedSites.add(data.countRejectedSites / updays);
+      acceptedRequests.add(data.countAcceptedRequests / updays);
+      rejectedRequests.add(data.countRejectedRequests / updays);
+    }
+    this._thirdPartyCookies.clear();
+  }
+};
+
+/**
+ * Data gathered on cookies that a third party site has attempted to set.
+ *
+ * Privacy note: the only data actually sent to the server is the size of
+ * the sets.
+ *
+ * @constructor
+ */
+let RejectStats = function() {
+  /**
+   * The set of all sites for which we have accepted third-party cookies.
+   */
+  this._acceptedSites = new Set();
+  /**
+   * The set of all sites for which we have rejected third-party cookies.
+   */
+  this._rejectedSites = new Set();
+  /**
+   * Total number of attempts to set a third-party cookie that have
+   * been accepted. Two accepted attempts on the same site will both
+   * augment this count.
+   */
+  this._acceptedRequests = 0;
+  /**
+   * Total number of attempts to set a third-party cookie that have
+   * been rejected. Two rejected attempts on the same site will both
+   * augment this count.
+   */
+  this._rejectedRequests = 0;
+};
+RejectStats.prototype = {
+  addAccepted: function(firstParty) {
+    this._acceptedSites.add(firstParty);
+    this._acceptedRequests++;
+  },
+  addRejected: function(firstParty) {
+    this._rejectedSites.add(firstParty);
+    this._rejectedRequests++;
+  },
+  get countAcceptedSites() {
+    return this._acceptedSites.size;
+  },
+  get countRejectedSites() {
+    return this._rejectedSites.size;
+  },
+  get countAcceptedRequests() {
+    return this._acceptedRequests;
+  },
+  get countRejectedRequests() {
+    return this._rejectedRequests;
+  }
+};
+
+/**
+ * Normalize a host to its eTLD + 1.
+ */
+function normalizeHost(host) {
+  return Services.eTLD.getBaseDomainFromHost(host);
+};
diff --git a/toolkit/components/telemetry/moz.build b/toolkit/components/telemetry/moz.build
index 81adbbacbe4..c1afc2a93e9 100644
--- a/toolkit/components/telemetry/moz.build
+++ b/toolkit/components/telemetry/moz.build
@@ -30,3 +30,7 @@ EXTRA_COMPONENTS += [
 EXTRA_PP_COMPONENTS += [
     'TelemetryPing.js',
 ]
+
+EXTRA_JS_MODULES += [
+    'ThirdPartyCookieProbe.jsm',
+]