--- a/browser/base/content/test/newtab/head.js
+++ b/browser/base/content/test/newtab/head.js
@@ -1,15 +1,24 @@
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 const PREF_NEWTAB_ENABLED = "browser.newtabpage.enabled";
 const PREF_NEWTAB_DIRECTORYSOURCE = "browser.newtabpage.directory.source";
+const PREF_NEWTAB_ACTIVITY_STREAM = "browser.newtabpage.activity-stream.enabled";
 
 Services.prefs.setBoolPref(PREF_NEWTAB_ENABLED, true);
+Services.prefs.setBoolPref(PREF_NEWTAB_ACTIVITY_STREAM, false);
+
+// Opens and closes a new tab to clear any existing preloaded ones. This is
+// necessary to prevent any left-over activity-stream preloaded new tabs from
+// affecting these tests.
+BrowserOpenTab();
+const initialTab = gBrowser.selectedTab;
+gBrowser.removeTab(initialTab);
 
 var tmp = {};
 Cu.import("resource://gre/modules/NewTabUtils.jsm", tmp);
 Cu.import("resource:///modules/DirectoryLinksProvider.jsm", tmp);
 Cu.import("resource://testing-common/PlacesTestUtils.jsm", tmp);
 Cc["@mozilla.org/moz/jssubscript-loader;1"]
   .getService(Ci.mozIJSSubScriptLoader)
   .loadSubScript("chrome://browser/content/sanitize.js", tmp);
@@ -73,16 +82,17 @@ registerCleanupFunction(function() {
 
   Object.keys(oldSize).forEach(prop => {
     if (oldSize[prop]) {
       gBrowser.contentWindow[prop] = oldSize[prop];
     }
   });
 
   Services.prefs.clearUserPref(PREF_NEWTAB_ENABLED);
+  Services.prefs.clearUserPref(PREF_NEWTAB_ACTIVITY_STREAM);
   Services.prefs.setCharPref(PREF_NEWTAB_DIRECTORYSOURCE, gOrigDirectorySource);
 
   return watchLinksChangeOnce();
 });
 
 function pushPrefs(...aPrefs) {
   return SpecialPowers.pushPrefEnv({"set": aPrefs});
 }

--- a/browser/components/newtab/tests/browser/browser_newtab_overrides.js
+++ b/browser/components/newtab/tests/browser/browser_newtab_overrides.js
@@ -1,21 +1,24 @@
 "use strict";
 
 let Cu = Components.utils;
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/Preferences.jsm");
+const PREF_NEWTAB_ACTIVITY_STREAM = "browser.newtabpage.activity-stream.enabled";
+
+Services.prefs.setBoolPref(PREF_NEWTAB_ACTIVITY_STREAM, false);
 
 XPCOMUtils.defineLazyServiceGetter(this, "aboutNewTabService",
                                    "@mozilla.org/browser/aboutnewtab-service;1",
                                    "nsIAboutNewTabService");
 
 registerCleanupFunction(function() {
-  Services.prefs.setBoolPref("browser.newtabpage.activity-stream.enabled", false);
+  Services.prefs.clearUserPref(PREF_NEWTAB_ACTIVITY_STREAM);
   aboutNewTabService.resetNewTabURL();
 });
 
 /*
  * Tests that the default newtab page is always returned when one types "about:newtab" in the URL bar,
  * even when overridden.
  */
 add_task(async function redirector_ignores_override() {