--- a/browser/components/preferences/in-content-new/tests/browser_advanced_update.js
+++ b/browser/components/preferences/in-content-new/tests/browser_advanced_update.js
@@ -109,32 +109,33 @@ add_task(async function() {
   gBrowser.removeCurrentTab();
 });
 
 add_task(async function() {
   await openPreferencesViaOpenPreferencesAPI("advanced", { leaveOpen: true });
   let doc = gBrowser.selectedBrowser.contentDocument;
 
   let showBtn = doc.getElementById("showUpdateHistory");
-  let dialogOverlay = doc.getElementById("dialogOverlay");
+  let dialogOverlay = content.gSubDialog._preloadDialog._overlay;
 
   // XXX: For unknown reasons, this mock cannot be loaded by
   // XPCOMUtils.defineLazyServiceGetter() called in aboutDialog-appUpdater.js.
   // It is registered here so that we could assert update history subdialog
   // without stopping the preferences advanced pane from loading.
   // See bug 1361929.
   mockUpdateManager.register();
 
   // Test the dialog window opens
   is(dialogOverlay.style.visibility, "", "The dialog should be invisible");
+  let promiseSubDialogLoaded = promiseLoadSubDialog("chrome://mozapps/content/update/history.xul");
   showBtn.doCommand();
-  await promiseLoadSubDialog("chrome://mozapps/content/update/history.xul");
+  await promiseSubDialogLoaded;
   is(dialogOverlay.style.visibility, "visible", "The dialog should be visible");
 
-  let dialogFrame = doc.getElementById("dialogFrame");
+  let dialogFrame = dialogOverlay.querySelector(".dialogFrame");
   let frameDoc = dialogFrame.contentDocument;
   let updates = frameDoc.querySelectorAll("update");
 
   // Test the update history numbers are correct
   is(updates.length, mockUpdateManager.updateCount, "The update count is incorrect.");
 
   // Test the updates are displayed correctly
   let update = null;
@@ -146,15 +147,15 @@ add_task(async function() {
     is(update.name, updateData.name + " (" + updateData.buildID + ")", "Wrong update name");
     is(update.type, updateData.type == "major" ? "New Version" : "Security Update", "Wrong update type");
     is(update.installDate, formatInstallDate(updateData.installDate), "Wrong update installDate");
     is(update.detailsURL, updateData.detailsURL, "Wrong update detailsURL");
     is(update.status, updateData.statusText, "Wrong update status");
   }
 
   // Test the dialog window closes
-  let closeBtn = doc.getElementById("dialogClose");
+  let closeBtn = dialogOverlay.querySelector(".dialogClose");
   closeBtn.doCommand();
   is(dialogOverlay.style.visibility, "", "The dialog should be invisible");
 
   mockUpdateManager.unregister();
   gBrowser.removeCurrentTab();
 });

--- a/browser/components/preferences/in-content-new/tests/browser_basic_rebuild_fonts_test.js
+++ b/browser/components/preferences/in-content-new/tests/browser_basic_rebuild_fonts_test.js
@@ -15,18 +15,19 @@ add_task(async function() {
   let fontFamily = Services.prefs.getCharPref("font.name." + defaultFontType + "." + langGroup);
   let fontFamilyField = doc.getElementById("defaultFont");
   is(fontFamilyField.value, fontFamily, "Font family should be set correctly.");
 
   let defaultFontSize = Services.prefs.getIntPref("font.size.variable." + langGroup);
   let fontSizeField = doc.getElementById("defaultFontSize");
   is(fontSizeField.value, defaultFontSize, "Font size should be set correctly.");
 
+  let promiseSubDialogLoaded = promiseLoadSubDialog("chrome://browser/content/preferences/fonts.xul");
   doc.getElementById("advancedFonts").click();
-  let win = await promiseLoadSubDialog("chrome://browser/content/preferences/fonts.xul");
+  let win = await promiseSubDialogLoaded;
   doc = win.document;
 
   // Simulate a dumb font backend.
   win.FontBuilder._enumerator = {
     _list: ["MockedFont1", "MockedFont2", "MockedFont3"],
     _defaultFont: null,
     EnumerateFonts(lang, type, list) {
       return this._list;

--- a/browser/components/preferences/in-content-new/tests/browser_cookies_exceptions.js
+++ b/browser/components/preferences/in-content-new/tests/browser_cookies_exceptions.js
@@ -334,18 +334,20 @@ var testRunner = {
         Services.prefs.clearUserPref("privacy.history.custom");
       });
 
       openPreferencesViaOpenPreferencesAPI("panePrivacy", {leaveOpen: true}).then(function() {
         let doc = gBrowser.contentDocument;
         let historyMode = doc.getElementById("historyMode");
         historyMode.value = "custom";
         historyMode.doCommand();
+
+        let promiseSubDialogLoaded =
+          promiseLoadSubDialog("chrome://browser/content/preferences/permissions.xul");
         doc.getElementById("cookieExceptions").doCommand();
 
-        let subDialogURL = "chrome://browser/content/preferences/permissions.xul";
-        promiseLoadSubDialog(subDialogURL).then(function(win) {
+        promiseSubDialogLoaded.then(function(win) {
           helperFunctions.windowLoad(win);
         });
       });
     });
   },
 };

--- a/browser/components/preferences/in-content-new/tests/browser_siteData.js
+++ b/browser/components/preferences/in-content-new/tests/browser_siteData.js
@@ -106,18 +106,18 @@ add_task(async function() {
   await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_QUOTA_USAGE_URL);
   await waitForEvent(gBrowser.selectedBrowser.contentWindow, "test-indexedDB-done");
   await BrowserTestUtils.removeTab(gBrowser.selectedTab);
 
   let updatedPromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatedPromise;
   await openSiteDataSettingsDialog();
-  let doc = gBrowser.selectedBrowser.contentDocument;
-  let dialogFrame = doc.getElementById("dialogFrame");
+  let dialog = content.gSubDialog._topDialog;
+  let dialogFrame = dialog._frame;
   let frameDoc = dialogFrame.contentDocument;
 
   let siteItems = frameDoc.getElementsByTagName("richlistitem");
   is(siteItems.length, 2, "Should list sites using quota usage or appcache");
 
   let appcacheSite = frameDoc.querySelector(`richlistitem[host="${TEST_OFFLINE_HOST}"]`);
   ok(appcacheSite, "Should list site using appcache");
 
@@ -275,18 +275,18 @@ add_task(async function() {
     },
   ];
 
   let updatePromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatePromise;
   await openSiteDataSettingsDialog();
 
-  let doc = gBrowser.selectedBrowser.contentDocument;
-  let dialogFrame = doc.getElementById("dialogFrame");
+  let dialog = content.gSubDialog._topDialog;
+  let dialogFrame = dialog._frame;
   let frameDoc = dialogFrame.contentDocument;
   let hostCol = frameDoc.getElementById("hostCol");
   let usageCol = frameDoc.getElementById("usageCol");
   let statusCol = frameDoc.getElementById("statusCol");
   let sitesList = frameDoc.getElementById("sitesList");
 
   // Test default sorting
   assertSortByUsage("descending");
@@ -401,17 +401,17 @@ add_task(async function() {
   let fakeHosts = mockSiteDataManager.fakeSites.map(site => site.principal.URI.host);
 
   let updatePromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatePromise;
   await openSiteDataSettingsDialog();
 
   let doc = gBrowser.selectedBrowser.contentDocument;
-  let frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  let frameDoc = content.gSubDialog._topDialog._frame.contentDocument;
   let searchBox = frameDoc.getElementById("searchBox");
 
   searchBox.value = "xyz";
   searchBox.doCommand();
   assertSitesListed(doc, fakeHosts.filter(host => host.includes("xyz")));
 
   searchBox.value = "bar";
   searchBox.doCommand();

--- a/browser/components/preferences/in-content-new/tests/browser_siteData2.js
+++ b/browser/components/preferences/in-content-new/tests/browser_siteData2.js
@@ -1,21 +1,21 @@
 "use strict";
 const { SiteDataManager } = Cu.import("resource:///modules/SiteDataManager.jsm", {});
 
 const REMOVE_DIALOG_URL = "chrome://browser/content/preferences/siteDataRemoveSelected.xul";
 const { DownloadUtils } = Cu.import("resource://gre/modules/DownloadUtils.jsm", {});
 
 function promiseSettingsDialogClose() {
   return new Promise(resolve => {
-    let doc = gBrowser.selectedBrowser.contentDocument;
-    let dialogOverlay = doc.getElementById("dialogOverlay");
-    let win = content.gSubDialog._frame.contentWindow;
-    win.addEventListener("unload", function unload() {
-      if (win.document.documentURI === "chrome://browser/content/preferences/siteDataSettings.xul") {
+    let win = gBrowser.selectedBrowser.contentWindow;
+    let dialogOverlay = win.gSubDialog._topDialog._overlay;
+    let dialogWin = win.gSubDialog._topDialog._frame.contentWindow;
+    dialogWin.addEventListener("unload", function unload() {
+      if (dialogWin.document.documentURI === "chrome://browser/content/preferences/siteDataSettings.xul") {
         isnot(dialogOverlay.style.visibility, "visible", "The Settings dialog should be hidden");
         resolve();
       }
     }, { once: true });
   });
 }
 
 // Test selecting and removing all sites one by one
@@ -50,80 +50,81 @@ add_task(async function() {
   ];
   let fakeHosts = mockSiteDataManager.fakeSites.map(site => site.principal.URI.host);
 
   let updatePromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatePromise;
   await openSiteDataSettingsDialog();
 
+  let win = gBrowser.selectedBrowser.contentWindow;
   let doc = gBrowser.selectedBrowser.contentDocument;
   let frameDoc = null;
   let saveBtn = null;
   let cancelBtn = null;
   let settingsDialogClosePromise = null;
 
   // Test the initial state
   assertSitesListed(doc, fakeHosts);
 
   // Test the "Cancel" button
   settingsDialogClosePromise = promiseSettingsDialogClose();
-  frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   cancelBtn = frameDoc.getElementById("cancel");
   removeAllSitesOneByOne();
   assertAllSitesNotListed();
   cancelBtn.doCommand();
   await settingsDialogClosePromise;
   await openSiteDataSettingsDialog();
   assertSitesListed(doc, fakeHosts);
 
   // Test the "Save Changes" button but cancelling save
   let cancelPromise = promiseAlertDialogOpen("cancel");
   settingsDialogClosePromise = promiseSettingsDialogClose();
-  frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   saveBtn = frameDoc.getElementById("save");
   removeAllSitesOneByOne();
   assertAllSitesNotListed();
   saveBtn.doCommand();
   await cancelPromise;
   await settingsDialogClosePromise;
   await openSiteDataSettingsDialog();
   assertSitesListed(doc, fakeHosts);
 
   // Test the "Save Changes" button and accepting save
   let acceptPromise = promiseAlertDialogOpen("accept");
   settingsDialogClosePromise = promiseSettingsDialogClose();
   updatePromise = promiseSiteDataManagerSitesUpdated();
-  frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   saveBtn = frameDoc.getElementById("save");
   removeAllSitesOneByOne();
   assertAllSitesNotListed();
   saveBtn.doCommand();
   await acceptPromise;
   await settingsDialogClosePromise;
   await updatePromise;
   await openSiteDataSettingsDialog();
   assertAllSitesNotListed();
 
   mockSiteDataManager.unregister();
   await BrowserTestUtils.removeTab(gBrowser.selectedTab);
 
   function removeAllSitesOneByOne() {
-    frameDoc = doc.getElementById("dialogFrame").contentDocument;
+    frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
     let removeBtn = frameDoc.getElementById("removeSelected");
     let sitesList = frameDoc.getElementById("sitesList");
     let sites = sitesList.getElementsByTagName("richlistitem");
     for (let i = sites.length - 1; i >= 0; --i) {
       sites[i].click();
       removeBtn.doCommand();
     }
   }
 
   function assertAllSitesNotListed() {
-    frameDoc = doc.getElementById("dialogFrame").contentDocument;
+    frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
     let removeBtn = frameDoc.getElementById("removeSelected");
     let removeAllBtn = frameDoc.getElementById("removeAll");
     let sitesList = frameDoc.getElementById("sitesList");
     let sites = sitesList.getElementsByTagName("richlistitem");
     is(sites.length, 0, "Should not list all sites");
     is(removeBtn.disabled, true, "Should disable the removeSelected button");
     is(removeAllBtn.disabled, true, "Should disable the removeAllBtn button");
   }
@@ -179,68 +180,69 @@ add_task(async function() {
   ];
   let fakeHosts = mockSiteDataManager.fakeSites.map(site => site.principal.URI.host);
 
   let updatePromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatePromise;
   await openSiteDataSettingsDialog();
 
+  let win = gBrowser.selectedBrowser.contentWindow;
   let doc = gBrowser.selectedBrowser.contentDocument;
   let frameDoc = null;
   let saveBtn = null;
   let cancelBtn = null;
   let removeDialogOpenPromise = null;
   let settingsDialogClosePromise = null;
 
   // Test the initial state
   assertSitesListed(doc, fakeHosts);
 
   // Test the "Cancel" button
   settingsDialogClosePromise = promiseSettingsDialogClose();
-  frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   cancelBtn = frameDoc.getElementById("cancel");
   removeSelectedSite(fakeHosts.slice(0, 2));
   assertSitesListed(doc, fakeHosts.slice(2));
   cancelBtn.doCommand();
   await settingsDialogClosePromise;
   await openSiteDataSettingsDialog();
   assertSitesListed(doc, fakeHosts);
 
   // Test the "Save Changes" button but canceling save
   removeDialogOpenPromise = promiseWindowDialogOpen("cancel", REMOVE_DIALOG_URL);
   settingsDialogClosePromise = promiseSettingsDialogClose();
-  frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   saveBtn = frameDoc.getElementById("save");
   removeSelectedSite(fakeHosts.slice(0, 2));
   assertSitesListed(doc, fakeHosts.slice(2));
   saveBtn.doCommand();
   await removeDialogOpenPromise;
   await settingsDialogClosePromise;
   await openSiteDataSettingsDialog();
   assertSitesListed(doc, fakeHosts);
 
   // Test the "Save Changes" button and accepting save
   removeDialogOpenPromise = promiseWindowDialogOpen("accept", REMOVE_DIALOG_URL);
   settingsDialogClosePromise = promiseSettingsDialogClose();
-  frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   saveBtn = frameDoc.getElementById("save");
   removeSelectedSite(fakeHosts.slice(0, 2));
   assertSitesListed(doc, fakeHosts.slice(2));
   saveBtn.doCommand();
   await removeDialogOpenPromise;
   await settingsDialogClosePromise;
   await openSiteDataSettingsDialog();
   assertSitesListed(doc, fakeHosts.slice(2));
 
   mockSiteDataManager.unregister();
   await BrowserTestUtils.removeTab(gBrowser.selectedTab);
 
   function removeSelectedSite(hosts) {
-    frameDoc = doc.getElementById("dialogFrame").contentDocument;
+    frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
     let removeBtn = frameDoc.getElementById("removeSelected");
     let sitesList = frameDoc.getElementById("sitesList");
     hosts.forEach(host => {
       let site = sitesList.querySelector(`richlistitem[host="${host}"]`);
       if (site) {
         site.click();
         removeBtn.doCommand();
       } else {
@@ -283,18 +285,19 @@ add_task(async function() {
   let fakeHosts = mockSiteDataManager.fakeSites.map(site => site.principal.URI.host);
 
   let updatePromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatePromise;
   await openSiteDataSettingsDialog();
 
   // Search "foo" to only list foo.com sites
+  let win = gBrowser.selectedBrowser.contentWindow;
   let doc = gBrowser.selectedBrowser.contentDocument;
-  let frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  let frameDoc = win.gSubDialog._topDialog._frame.contentDocument;
   let searchBox = frameDoc.getElementById("searchBox");
   searchBox.value = "xyz";
   searchBox.doCommand();
   assertSitesListed(doc, fakeHosts.filter(host => host.includes("xyz")));
 
   // Test only removing all visible sites listed
   updatePromise = promiseSiteDataManagerSitesUpdated();
   let acceptRemovePromise = promiseWindowDialogOpen("accept", REMOVE_DIALOG_URL);
@@ -344,17 +347,18 @@ add_task(async function() {
       persisted: false
     },
   ];
 
   let updatedPromise = promiseSiteDataManagerSitesUpdated();
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
   await updatedPromise;
   await openSiteDataSettingsDialog();
-  let dialogFrame = gBrowser.selectedBrowser.contentDocument.getElementById("dialogFrame");
+  let win = gBrowser.selectedBrowser.contentWindow;
+  let dialogFrame = win.gSubDialog._topDialog._frame;
   let frameDoc = dialogFrame.contentDocument;
 
   let siteItems = frameDoc.getElementsByTagName("richlistitem");
   is(siteItems.length, 1, "Should group sites across scheme, port and origin attributes");
 
   let expected = "account.xyz.com";
   let host = siteItems[0].getAttribute("host");
   is(host, expected, "Should group and list sites by host");

--- a/browser/components/preferences/in-content-new/tests/head.js
+++ b/browser/components/preferences/in-content-new/tests/head.js
@@ -40,39 +40,39 @@ function open_preferences(aCallback) {
 function openAndLoadSubDialog(aURL, aFeatures = null, aParams = null, aClosingCallback = null) {
   let promise = promiseLoadSubDialog(aURL);
   content.gSubDialog.open(aURL, aFeatures, aParams, aClosingCallback);
   return promise;
 }
 
 function promiseLoadSubDialog(aURL) {
   return new Promise((resolve, reject) => {
-    content.gSubDialog._frame.addEventListener("load", function load(aEvent) {
-      if (aEvent.target.contentWindow.location == "about:blank")
+    content.gSubDialog._dialogStack.addEventListener("dialogopen", function dialogopen(aEvent) {
+      if (aEvent.detail.dialog._frame.contentWindow.location == "about:blank")
         return;
-      content.gSubDialog._frame.removeEventListener("load", load);
+      content.gSubDialog._dialogStack.removeEventListener("dialogopen", dialogopen);
 
-      is(content.gSubDialog._frame.contentWindow.location.toString(), aURL,
+      is(aEvent.detail.dialog._frame.contentWindow.location.toString(), aURL,
          "Check the proper URL is loaded");
 
       // Check visibility
-      is_element_visible(content.gSubDialog._overlay, "Overlay is visible");
+      is_element_visible(aEvent.detail.dialog._overlay, "Overlay is visible");
 
       // Check that stylesheets were injected
-      let expectedStyleSheetURLs = content.gSubDialog._injectedStyleSheets.slice(0);
-      for (let styleSheet of content.gSubDialog._frame.contentDocument.styleSheets) {
+      let expectedStyleSheetURLs = aEvent.detail.dialog._injectedStyleSheets.slice(0);
+      for (let styleSheet of aEvent.detail.dialog._frame.contentDocument.styleSheets) {
         let i = expectedStyleSheetURLs.indexOf(styleSheet.href);
         if (i >= 0) {
           info("found " + styleSheet.href);
           expectedStyleSheetURLs.splice(i, 1);
         }
       }
       is(expectedStyleSheetURLs.length, 0, "All expectedStyleSheetURLs should have been found");
 
-      resolve(content.gSubDialog._frame.contentWindow);
+      resolve(aEvent.detail.dialog._frame.contentWindow);
     });
   });
 }
 
 /**
  * Waits a specified number of miliseconds for a specified event to be
  * fired on a specified element.
  *
@@ -188,28 +188,28 @@ function addPersistentStoragePerm(origin
 
 function promiseSiteDataManagerSitesUpdated() {
   return TestUtils.topicObserved("sitedatamanager:sites-updated", () => true);
 }
 
 function openSiteDataSettingsDialog() {
   let doc = gBrowser.selectedBrowser.contentDocument;
   let settingsBtn = doc.getElementById("siteDataSettings");
-  let dialogOverlay = doc.getElementById("dialogOverlay");
+  let dialogOverlay = content.gSubDialog._preloadDialog._overlay;
   let dialogLoadPromise = promiseLoadSubDialog("chrome://browser/content/preferences/siteDataSettings.xul");
   let dialogInitPromise = TestUtils.topicObserved("sitedata-settings-init", () => true);
   let fullyLoadPromise = Promise.all([ dialogLoadPromise, dialogInitPromise ]).then(() => {
     is(dialogOverlay.style.visibility, "visible", "The Settings dialog should be visible");
   });
   settingsBtn.doCommand();
   return fullyLoadPromise;
 }
 
 function assertSitesListed(doc, hosts) {
-  let frameDoc = doc.getElementById("dialogFrame").contentDocument;
+  let frameDoc = content.gSubDialog._topDialog._frame.contentDocument;
   let removeBtn = frameDoc.getElementById("removeSelected");
   let removeAllBtn = frameDoc.getElementById("removeAll");
   let sitesList = frameDoc.getElementById("sitesList");
   let totalSitesNumber = sitesList.getElementsByTagName("richlistitem").length;
   is(totalSitesNumber, hosts.length, "Should list the right sites number");
   hosts.forEach(host => {
     let site = sitesList.querySelector(`richlistitem[host="${host}"]`);
     ok(site, `Should list the site of ${host}`);