--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -1410,8 +1410,10 @@ pref("media.webspeech.synth.enabled", tr
 pref("browser.esedbreader.loglevel", "Error");
 
 pref("browser.laterrun.enabled", false);
 
 // Enable browser frames for use on desktop.  Only exposed to chrome callers.
 pref("dom.mozBrowserFramesEnabled", true);
 
 pref("extensions.pocket.enabled", true);
+
+pref("network.captive-portal-service.enabled", true);

--- a/browser/modules/test/browser.ini
+++ b/browser/modules/test/browser.ini
@@ -1,16 +1,17 @@
 [DEFAULT]
 support-files =
   head.js
 
 [browser_BrowserUITelemetry_buckets.js]
 [browser_BrowserUITelemetry_defaults.js]
 [browser_BrowserUITelemetry_sidebar.js]
 [browser_BrowserUITelemetry_syncedtabs.js]
+[browser_CaptivePortalWatcher.js]
 [browser_ProcessHangNotifications.js]
 skip-if = !e10s
 [browser_ContentSearch.js]
 support-files =
   contentSearch.js
   contentSearchBadImage.xml
   contentSearchSuggestions.sjs
   contentSearchSuggestions.xml

new file mode 100644
--- /dev/null
+++ b/browser/modules/test/browser_CaptivePortalWatcher.js
@@ -0,0 +1,175 @@
+/* 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/. */
+
+Components.utils.import("resource:///modules/RecentWindow.jsm");
+
+const CANONICAL_CONTENT = "success";
+const CANONICAL_URL = "data:text/plain;," + CANONICAL_CONTENT;
+const CANONICAL_URL_REDIRECTED = "data:text/plain;,redirected";
+
+Services.prefs.setCharPref("captivedetect.canonicalURL", CANONICAL_URL);
+Services.prefs.setCharPref("captivedetect.canonicalContent", CANONICAL_CONTENT);
+
+/* We can't close the original window opened by mochitest without failing, so
+ * override RecentWindow.getMostRecentBrowserWindow to make CaptivePortalWatcher
+ * think there's no window. */
+function* portalDetectedNoBrowserWindow() {
+  let getMostRecentBrowserWindow = RecentWindow.getMostRecentBrowserWindow;
+  RecentWindow.getMostRecentBrowserWindow = () => {};
+  Services.obs.notifyObservers(null, "captive-portal-login", null);
+  RecentWindow.getMostRecentBrowserWindow = getMostRecentBrowserWindow;
+}
+
+function* openWindowAndWaitForPortalTab() {
+  let win = yield BrowserTestUtils.openNewBrowserWindow();
+  let tab = yield BrowserTestUtils.waitForNewTab(win.gBrowser, CANONICAL_URL);
+  is(win.gBrowser.selectedTab, tab,
+    "The captive portal tab should be open and selected in the new window.");
+  return win;
+}
+
+function freePortal(aSuccess) {
+  Services.obs.notifyObservers(null,
+    "captive-portal-login-" + (aSuccess ? "success" : "abort"), null);
+}
+
+// Each of the test cases below is run twice: once for login-success and once
+// for login-abort (aSuccess set to true and false respectively).
+var testCasesForBothSuccessAndAbort = [
+  /* A portal is detected when there's no browser window,
+   * then a browser window is opened, then the portal is freed.
+   * The portal tab should be added and focused when the window is
+   * opened, and closed automatically when the success event is fired. */
+  function* (aSuccess) {
+    yield portalDetectedNoBrowserWindow();
+    let win = yield openWindowAndWaitForPortalTab();
+    freePortal(aSuccess);
+    is(win.gBrowser.tabs.length, 1,
+      "The captive portal tab should have been closed.");
+    yield BrowserTestUtils.closeWindow(win);
+  },
+
+  /* A portal is detected when there's no browser window, and the
+   * portal is freed before a browser window is opened. No portal
+   * tab should be added when a browser window is opened. */
+  function* (aSuccess) {
+    yield portalDetectedNoBrowserWindow();
+    freePortal(aSuccess);
+    let win = yield BrowserTestUtils.openNewBrowserWindow();
+    // Wait for a while to make sure no tab is opened.
+    yield new Promise(resolve => {
+      setTimeout(resolve, 1000);
+    });
+    is(win.gBrowser.tabs.length, 1,
+      "No captive portal tab should have been opened.");
+    yield BrowserTestUtils.closeWindow(win);
+  },
+
+  /* A portal is detected when a browser window has focus. A portal tab should be
+   * opened in the background in the focused browser window. If the portal is
+   * freed when the tab isn't focused, the tab should be closed automatically. */
+  function* (aSuccess) {
+    let win = RecentWindow.getMostRecentBrowserWindow();
+    let p = BrowserTestUtils.waitForNewTab(win.gBrowser, CANONICAL_URL);
+    Services.obs.notifyObservers(null, "captive-portal-login", null);
+    let tab = yield p;
+    isnot(win.gBrowser.selectedTab, tab,
+      "The captive portal tab should be open in the background in the current window.");
+    freePortal(aSuccess);
+    is(win.gBrowser.tabs.length, 1,
+      "The portal tab should have been closed.");
+  },
+
+  /* A portal is detected when a browser window has focus. A portal tab should be
+   * opened in the background in the focused browser window. If the portal is
+   * freed when the tab has focus, the tab should be closed automatically. */
+  function* (aSuccess) {
+    let win = RecentWindow.getMostRecentBrowserWindow();
+    let p = BrowserTestUtils.waitForNewTab(win.gBrowser, CANONICAL_URL);
+    Services.obs.notifyObservers(null, "captive-portal-login", null);
+    let tab = yield p;
+    isnot(win.gBrowser.selectedTab, tab,
+      "The captive portal tab should be open in the background in the current window.");
+    win.gBrowser.selectedTab = tab;
+    freePortal(aSuccess);
+    is(win.gBrowser.tabs.length, 1,
+      "The portal tab should have been closed.");
+  },
+];
+
+var singleRunTestCases = [
+  /* A portal is detected when there's no browser window,
+   * then a browser window is opened, and the portal is logged into
+   * and redirects to a different page. The portal tab should be added
+   * and focused when the window is opened, and left open after login
+   * since it redirected. */
+  function* () {
+    yield portalDetectedNoBrowserWindow();
+    let win = yield openWindowAndWaitForPortalTab();
+    let browser = win.gBrowser.selectedTab.linkedBrowser;
+    let loadPromise =
+      BrowserTestUtils.browserLoaded(browser, false, CANONICAL_URL_REDIRECTED);
+    BrowserTestUtils.loadURI(browser, CANONICAL_URL_REDIRECTED);
+    yield loadPromise;
+    freePortal(true);
+    is(win.gBrowser.tabs.length, 2,
+      "The captive portal tab should not have been closed.");
+    yield BrowserTestUtils.closeWindow(win);
+  },
+
+  /* A portal is detected when a browser window has focus. A portal tab should be
+   * opened in the background in the focused browser window. If the portal is
+   * freed when the tab isn't focused, the tab should be closed automatically,
+   * even if the portal has redirected to a URL other than CANONICAL_URL. */
+  function* () {
+    let win = RecentWindow.getMostRecentBrowserWindow();
+    let p = BrowserTestUtils.waitForNewTab(win.gBrowser, CANONICAL_URL);
+    Services.obs.notifyObservers(null, "captive-portal-login", null);
+    let tab = yield p;
+    isnot(win.gBrowser.selectedTab, tab,
+      "The captive portal tab should be open in the background in the current window.");
+    let browser = tab.linkedBrowser;
+    let loadPromise =
+      BrowserTestUtils.browserLoaded(browser, false, CANONICAL_URL_REDIRECTED);
+    BrowserTestUtils.loadURI(browser, CANONICAL_URL_REDIRECTED);
+    yield loadPromise;
+    freePortal(true);
+    is(win.gBrowser.tabs.length, 1,
+      "The portal tab should have been closed.");
+  },
+
+  /* A portal is detected when a browser window has focus. A portal tab should be
+   * opened in the background in the focused browser window. If the portal is
+   * freed when the tab has focus, and it has redirected to another page, the
+   * tab should be kept open. */
+  function* () {
+    let win = RecentWindow.getMostRecentBrowserWindow();
+    let p = BrowserTestUtils.waitForNewTab(win.gBrowser, CANONICAL_URL);
+    Services.obs.notifyObservers(null, "captive-portal-login", null);
+    let tab = yield p;
+    isnot(win.gBrowser.selectedTab, tab,
+      "The captive portal tab should be open in the background in the current window.");
+    win.gBrowser.selectedTab = tab;
+    let browser = tab.linkedBrowser;
+    let loadPromise =
+      BrowserTestUtils.browserLoaded(browser, false, CANONICAL_URL_REDIRECTED);
+    BrowserTestUtils.loadURI(browser, CANONICAL_URL_REDIRECTED);
+    yield loadPromise;
+    freePortal(true);
+    is(win.gBrowser.tabs.length, 2,
+      "The portal tab should not have been closed.");
+    win.gBrowser.removeTab(tab);
+  },
+];
+
+testCasesForBothSuccessAndAbort.forEach(aCase => {
+  add_task(function* () {
+    yield aCase(true);
+    yield aCase(false);
+  });
+});
+
+singleRunTestCases.forEach(aCase => {
+  add_task(aCase);
+});