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diff --git a/toolkit/components/telemetry/ThirdPartyCookieProbe.jsm b/toolkit/components/telemetry/ThirdPartyCookieProbe.jsm
new file mode 100644
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+++ b/toolkit/components/telemetry/ThirdPartyCookieProbe.jsm
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+/* 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";
+
+var Ci = Components.interfaces;
+var Cu = Components.utils;
+var Cr = Components.results;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/Services.jsm", this);
+
+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
+      // Note: nsCookieService passes "?" if the issuer is unknown.  Avoid
+      //       normalizing in this case since its not a valid URI.
+      let firstParty = (referrer === "?") ? referrer : 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) {
+      if (ex instanceof Ci.nsIXPCException) {
+        if (ex.result == Cr.NS_ERROR_HOST_IS_IP_ADDRESS ||
+            ex.result == Cr.NS_ERROR_INSUFFICIENT_DOMAIN_LEVELS) {
+          return;
+        }
+      }
+      // Other errors should not remain silent.
+      Services.console.logStringMessage("ThirdPartyCookieProbe: Uncaught error " + ex + "\n" + ex.stack);
+    }
+  },
+
+  /**
+   * Clear internal data, fill up corresponding histograms.
+   *
+   * @param {number} aNow (optional, used for testing purposes only)
+   * The current instant. Used to make tests time-independent.
+   */
+  flush: function(aNow = Date.now()) {
+    let updays = (aNow - this._latestFlush) / MILLISECONDS_PER_DAY;
+    if (updays <= 0) {
+      // Unlikely, but regardless, don't risk division by zero
+      // or weird stuff.
+      return;
+    }
+    this._latestFlush = aNow;
+    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
+ */
+var 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);
+}