diff --git a/browser/components/originattributes/test/browser/browser_httpauth.js b/browser/components/originattributes/test/browser/browser_httpauth.js
index 61d429349949..7e9a990a9d52 100644
--- a/browser/components/originattributes/test/browser/browser_httpauth.js
+++ b/browser/components/originattributes/test/browser/browser_httpauth.js
@@ -1,5 +1,14 @@
 let { HttpServer } = ChromeUtils.import("resource://testing-common/httpd.js");
 
+let authPromptModalType = Services.prefs.getIntPref(
+  "prompts.modalType.httpAuth"
+);
+
+let commonDialogEnabled =
+  authPromptModalType === Services.prompt.MODAL_TYPE_WINDOW ||
+  (authPromptModalType === Services.prompt.MODAL_TYPE_TAB &&
+    Services.prefs.getBoolPref("prompts.tabChromePromptSubDialog"));
+
 let server = new HttpServer();
 server.registerPathHandler("/file.html", fileHandler);
 server.start(-1);
@@ -27,17 +36,27 @@ function fileHandler(metadata, response) {
 }
 
 function onCommonDialogLoaded(subject) {
+  let dialog;
+  if (commonDialogEnabled) {
+    dialog = subject.Dialog;
+  } else {
+    let promptBox =
+      subject.ownerGlobal.gBrowser.selectedBrowser.tabModalPromptBox;
+    dialog = promptBox.getPrompt(subject).Dialog;
+  }
   // Submit random account and password
-  let dialog = subject.Dialog;
   dialog.ui.loginTextbox.setAttribute("value", Math.random());
   dialog.ui.password1Textbox.setAttribute("value", Math.random());
   dialog.ui.button0.click();
 }
 
-Services.obs.addObserver(onCommonDialogLoaded, "common-dialog-loaded");
+let authPromptTopic = commonDialogEnabled
+  ? "common-dialog-loaded"
+  : "tabmodal-dialog-loaded";
+Services.obs.addObserver(onCommonDialogLoaded, authPromptTopic);
 
 registerCleanupFunction(() => {
-  Services.obs.removeObserver(onCommonDialogLoaded, "common-dialog-loaded");
+  Services.obs.removeObserver(onCommonDialogLoaded, authPromptTopic);
   server.stop(() => {
     server = null;
   });
diff --git a/toolkit/components/passwordmgr/test/browser/browser_basicAuth_rateLimit.js b/toolkit/components/passwordmgr/test/browser/browser_basicAuth_rateLimit.js
index d29b485ab07c..6046c71e0eb7 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_basicAuth_rateLimit.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_basicAuth_rateLimit.js
@@ -5,34 +5,14 @@
 // This tests that the basic auth dialog can not be used for DOS attacks
 // and that the protections are reset on user-initiated navigation/reload.
 
-const PROMPT_URL = "chrome://global/content/commonDialog.xhtml";
+let promptModalType = Services.prefs.getIntPref("prompts.modalType.httpAuth");
+
 function promiseAuthWindowShown() {
-  return new Promise(resolve => {
-    let listener = {
-      onOpenWindow(xulWin) {
-        let domwindow = xulWin.docShell.domWindow;
-        waitForFocus(() => {
-          is(
-            domwindow.document.location.href,
-            PROMPT_URL,
-            "Should have seen a prompt window"
-          );
-          is(
-            domwindow.args.promptType,
-            "promptUserAndPass",
-            "Should be an authenticate prompt"
-          );
-          domwindow.document.getElementById("commonDialog").cancelDialog();
-          Services.wm.removeListener(listener);
-          resolve();
-        }, domwindow);
-      },
-
-      onCloseWindow() {},
-    };
-
-    Services.wm.addListener(listener);
-  });
+  return PromptTestUtils.handleNextPrompt(
+    window,
+    { modalType: promptModalType, promptType: "promptUserAndPass" },
+    { buttonNumClick: 1 }
+  );
 }
 
 add_task(async function test() {
diff --git a/toolkit/components/passwordmgr/test/browser/browser_basicAuth_switchTab.js b/toolkit/components/passwordmgr/test/browser/browser_basicAuth_switchTab.js
index 100f7411cc25..1625217c0f44 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_basicAuth_switchTab.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_basicAuth_switchTab.js
@@ -2,47 +2,33 @@
  * 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/. */
 
-const PROMPT_URL = "chrome://global/content/commonDialog.xhtml";
+let modalType = Services.prefs.getIntPref("prompts.modalType.httpAuth");
 
 add_task(async function test() {
-  await new Promise(resolve => {
-    let tab = BrowserTestUtils.addTab(gBrowser);
-    isnot(tab, gBrowser.selectedTab, "New tab shouldn't be selected");
+  let tab = BrowserTestUtils.addTab(gBrowser);
+  isnot(tab, gBrowser.selectedTab, "New tab shouldn't be selected");
 
-    let listener = {
-      onOpenWindow(xulWin) {
-        var domwindow = xulWin.docShell.domWindow;
-        waitForFocus(() => {
-          is(
-            domwindow.document.location.href,
-            PROMPT_URL,
-            "Should have seen a prompt window"
-          );
-          is(
-            domwindow.args.promptType,
-            "promptUserAndPass",
-            "Should be an authenticate prompt"
-          );
-
-          is(gBrowser.selectedTab, tab, "Should have selected the new tab");
-
-          domwindow.document.getElementById("commonDialog").cancelDialog();
-        }, domwindow);
-      },
-
-      onCloseWindow() {},
-    };
-
-    Services.wm.addListener(listener);
-    registerCleanupFunction(() => {
-      Services.wm.removeListener(listener);
-      gBrowser.removeTab(tab);
-    });
-
-    BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => finish());
-    BrowserTestUtils.loadURI(
-      tab.linkedBrowser,
-      "http://example.com/browser/toolkit/components/passwordmgr/test/browser/authenticate.sjs"
-    );
+  let authPromptShown = PromptTestUtils.waitForPrompt(tab.linkedBrowser, {
+    modalType,
+    promptType: "promptUserAndPass",
   });
+
+  let loadPromise = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  BrowserTestUtils.loadURI(
+    tab.linkedBrowser,
+    "http://example.com/browser/toolkit/components/passwordmgr/test/browser/authenticate.sjs"
+  );
+
+  // Wait for the basic auth prompt
+  let dialog = await authPromptShown;
+
+  is(gBrowser.selectedTab, tab, "Should have selected the new tab");
+
+  // Cancel the auth prompt
+  PromptTestUtils.handlePrompt(dialog, { buttonNumClick: 1 });
+
+  // After closing the prompt the load should finish
+  await loadPromise;
+
+  gBrowser.removeTab(tab);
 });
diff --git a/toolkit/components/passwordmgr/test/browser/browser_private_window.js b/toolkit/components/passwordmgr/test/browser/browser_private_window.js
index a77b98cf2fd8..52bad4dd01c1 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_private_window.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_private_window.js
@@ -74,14 +74,13 @@ async function loadAccessRestrictedURL(browser, url, username, password) {
   let browserLoaded = BrowserTestUtils.browserLoaded(browser);
   BrowserTestUtils.loadURI(browser, url);
 
-  let promptDoc = await waitForAuthPrompt();
-  let dialogUI = promptDoc.defaultView.Dialog.ui;
-  ok(dialogUI, "Got expected HTTP auth dialog Dialog.ui");
+  // Wait for the auth prompt, enter the login details and close the prompt
+  await PromptTestUtils.handleNextPrompt(
+    browser,
+    { modalType: authPromptModalType, promptType: "promptUserAndPass" },
+    { buttonNumClick: 0, loginInput: username, passwordInput: password }
+  );
 
-  // fill and submit the dialog form
-  dialogUI.loginTextbox.value = username;
-  dialogUI.password1Textbox.value = password;
-  promptDoc.getElementById("commonDialog").acceptDialog();
   await SimpleTest.promiseFocus(browser.ownerGlobal);
   await browserLoaded;
 }
@@ -107,11 +106,13 @@ const authUrl = `https://example.com/${DIRECTORY_PATH}authenticate.sjs`;
 
 let normalWin;
 let privateWin;
+let authPromptModalType;
 
 // XXX: Note that tasks are currently run in sequence. Some tests may assume the state
 // resulting from successful or unsuccessful logins in previous tasks
 
 add_task(async function test_setup() {
+  authPromptModalType = Services.prefs.getIntPref("prompts.modalType.httpAuth");
   normalWin = await BrowserTestUtils.openNewBrowserWindow({ private: false });
   privateWin = await BrowserTestUtils.openNewBrowserWindow({ private: true });
   Services.logins.removeAllLogins();
diff --git a/toolkit/components/passwordmgr/test/browser/head.js b/toolkit/components/passwordmgr/test/browser/head.js
index e5415ae7a29b..015cedf9515e 100644
--- a/toolkit/components/passwordmgr/test/browser/head.js
+++ b/toolkit/components/passwordmgr/test/browser/head.js
@@ -7,6 +7,7 @@ const { LoginManagerParent } = ChromeUtils.import(
 ChromeUtils.import("resource://testing-common/LoginTestUtils.jsm", this);
 ChromeUtils.import("resource://testing-common/ContentTaskUtils.jsm", this);
 ChromeUtils.import("resource://testing-common/TelemetryTestUtils.jsm", this);
+ChromeUtils.import("resource://testing-common/PromptTestUtils.jsm", this);
 
 add_task(async function common_initialize() {
   await SpecialPowers.pushPrefEnv({
diff --git a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
index 81695766a551..d9159d54f56c 100644
--- a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
+++ b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
@@ -32,6 +32,23 @@ const {
 const LOGIN_FIELD_UTILS = LoginTestUtils.loginField;
 const TESTS_DIR = "/tests/toolkit/components/passwordmgr/test/";
 
+// Depending on pref state we either show auth prompts as windows or on tab level.
+let authPromptModalType = SpecialPowers.Services.prompt.MODAL_TYPE_WINDOW;
+if (SpecialPowers.Services.prefs.getBoolPref("prompts.tab_modal.enabled")) {
+  authPromptModalType = SpecialPowers.Services.prefs.getIntPref(
+    "prompts.modalType.httpAuth"
+  );
+}
+
+// Whether the auth prompt is a commonDialog.xhtml or a TabModalPrompt
+let authPromptIsCommonDialog =
+  authPromptModalType === SpecialPowers.Services.prompt.MODAL_TYPE_WINDOW ||
+  (authPromptModalType === SpecialPowers.Services.prompt.MODAL_TYPE_TAB &&
+    SpecialPowers.Services.prefs.getBoolPref(
+      "prompts.tabChromePromptSubDialog",
+      false
+    ));
+
 /**
  * Returns the element with the specified |name| attribute.
  */
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html b/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html
index 7bb27e62fb84..f28fb31aae08 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html
@@ -119,12 +119,30 @@
         sendAsyncMessage("prepareForNextTestDone");
       });
 
-      let dialogObserverTopic = "common-dialog-loaded";
+      let modalType = Services.prefs.getIntPref(
+        "prompts.modalType.httpAuth"
+      );
+      let authPromptIsCommonDialog =
+        modalType === Services.prompt.MODAL_TYPE_WINDOW
+          || (modalType === Services.prompt.MODAL_TYPE_TAB
+            && Services.prefs.getBoolPref(
+        "prompts.tabChromePromptSubDialog",
+        false
+      ));
+
+      let dialogObserverTopic = authPromptIsCommonDialog
+        ? "common-dialog-loaded" : "tabmodal-dialog-loaded";
 
       function dialogObserver(subj, topic, data) {
-        subj.Dialog.ui.prompt.document
+        if (authPromptIsCommonDialog) {
+          subj.Dialog.ui.prompt.document
           .getElementById("commonDialog")
           .acceptDialog();
+        } else {
+          let prompt = subj.ownerGlobal.gBrowser.selectedBrowser
+            .tabModalPromptBox.getPrompt(subj);
+          prompt.Dialog.ui.button0.click(); // Accept button
+        }
         sendAsyncMessage("promptAccepted");
       }
 
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_prompt_async.html b/toolkit/components/passwordmgr/test/mochitest/test_prompt_async.html
index d0bd3b020380..0cac26cf1c0e 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_prompt_async.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_prompt_async.html
@@ -16,9 +16,8 @@
       const EXAMPLE_ORG = "http://example.org/tests/toolkit/components/passwordmgr/test/mochitest/";
       let mozproxyOrigin;
 
-      // Force parent to not look for tab-modal prompts, as they're not used for auth prompts.
-      // Used by prompt_common.js.
-      modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+      // Let prompt_common know what kind of modal type is enabled for auth prompts.
+      modalType = authPromptModalType;
 
       // These are magically defined on the window due to the iframe IDs
       /* global iframe1, iframe2a, iframe2b */
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_prompt_http.html b/toolkit/components/passwordmgr/test/mochitest/test_prompt_http.html
index 9ad5a676b513..0222c58968ee 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_prompt_http.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_prompt_http.html
@@ -19,8 +19,8 @@
 <script class="testbody" type="text/javascript">
 var iframe = document.getElementById("iframe");
 
-// Force parent to not look for tab-modal prompts, as they're not used for auth prompts.
-modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+// Let prompt_common know what kind of modal type is enabled for auth prompts.
+modalType = authPromptModalType;
 
 const AUTHENTICATE_PATH = new URL("authenticate.sjs", window.location.href).pathname;
 
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_prompt_noWindow.html b/toolkit/components/passwordmgr/test/mochitest/test_prompt_noWindow.html
index f2df86f590c8..2c30efb0930d 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_prompt_noWindow.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_prompt_noWindow.html
@@ -17,8 +17,8 @@
 <pre id="test">
 <script class="testbody" type="text/javascript">
 
-// Force parent to not look for tab-modal prompts, as they're not used for auth prompts.
-modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+// Let prompt_common know what kind of modal type is enabled for auth prompts.
+modalType = authPromptModalType;
 
 add_task(async function setup() {
   let loginAddedPromise = promiseStorageChanged(["addLogin"]);
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth.html b/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth.html
index 2e3f20fb33f6..e7d501f44cce 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth.html
@@ -30,8 +30,8 @@ var authinfo = {
   realm: "",
 };
 
-// Force parent to not look for tab-modal prompts, as they're not used for auth prompts.
-modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+// Let prompt_common know what kind of modal type is enabled for auth prompts.
+modalType = authPromptModalType;
 
 let prompterParent = runInParent(() => {
   const promptFac = Cc["@mozilla.org/passwordmanager/authpromptfactory;1"].
@@ -40,6 +40,7 @@ let prompterParent = runInParent(() => {
   const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
   let chromeWin = Services.wm.getMostRecentWindow("navigator:browser");
   let prompter2 = promptFac.getPrompt(chromeWin, Ci.nsIAuthPrompt2);
+  prompter2.QueryInterface(Ci.nsILoginManagerAuthPrompter).browser = chromeWin.gBrowser.selectedBrowser;
 
   let channels = {};
   channels.channel1 = Services.io.newChannel("http://example.com",
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth_proxy.html b/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth_proxy.html
index 85a5ca0f711a..75e29a4a39c4 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth_proxy.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_prompt_promptAuth_proxy.html
@@ -40,7 +40,8 @@ var proxyAuthinfo = {
 };
 
 // Force parent to not look for tab-modal prompts, as they're not used for auth prompts.
-modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+// Let prompt_common know what kind of modal type is enabled for auth prompts.
+modalType = authPromptModalType;
 
 const Cc_promptFac = Cc["@mozilla.org/passwordmanager/authpromptfactory;1"];
 ok(Cc_promptFac != null, "Access Cc[@mozilla.org/passwordmanager/authpromptfactory;1]");
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_xhr.html b/toolkit/components/passwordmgr/test/mochitest/test_xhr.html
index 2fe62b4a28a4..7560731ea8d0 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_xhr.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_xhr.html
@@ -40,8 +40,8 @@ function xhrLoad(xmlDoc) {
   return {username, password, authok};
 }
 
-// Force parent to not look for tab-modal prompts, as they're not used for auth prompts.
-modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+// Let prompt_common know what kind of modal type is enabled for auth prompts.
+modalType = authPromptModalType;
 
 let prompterParent = runInParent(() => {
   const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
@@ -131,15 +131,19 @@ add_task(async function test2() {
     checked: false,
     focused: "textField",
     defButton: "button0",
-    // Check that the dialog is modal, chrome and dependent;
-    // We can't just check window.opener because that'll be
-    // a content window, which therefore isn't exposed (it'll lie and
-    // be null).
-    chrome: true,
-    dialog: true,
-    chromeDependent: true,
-    isWindowModal: true,
   };
+
+  // For window prompts check that the dialog is modal, chrome and dependent;
+  // We can't just check window.opener because that'll be
+  // a content window, which therefore isn't exposed (it'll lie and
+  // be null).
+  if (authPromptModalType === SpecialPowers.Services.prompt.MODAL_TYPE_WINDOW) {
+    state.chrome = true;
+    state.dialog = true;
+    state.chromeDependent = true;
+    state.isWindowModal = true;
+  }
+
   let action = {
     buttonClick: "ok",
   };
diff --git a/toolkit/components/prompts/test/test_subresources_prompts.html b/toolkit/components/prompts/test/test_subresources_prompts.html
index 9c30c387b9a5..b52c9fbfae09 100644
--- a/toolkit/components/prompts/test/test_subresources_prompts.html
+++ b/toolkit/components/prompts/test/test_subresources_prompts.html
@@ -33,6 +33,15 @@ var iframe1Loaded = onloadPromiseFor("iframe_diff_origin");
 var iframe2Loaded = onloadPromiseFor("iframe_same_origin");
 var iframe_prompt = document.getElementById("iframe_prompt");
 
+// Depending on pref state we either show auth prompts as windows or on tab level.
+let authPromptModalType = SpecialPowers.Services.prompt.MODAL_TYPE_WINDOW;
+if (SpecialPowers.Services.prefs.getBoolPref("prompts.tab_modal.enabled")) {
+  authPromptModalType = SpecialPowers.Services.prefs.getIntPref(
+    "prompts.modalType.httpAuth"
+  );
+}
+
+
 add_task(async function runTest() {
   // This test depends on tab modal prompts being enabled.
   if (!tabModalPromptEnabled) {
@@ -101,12 +110,11 @@ add_task(async function runTest() {
 });
 
 add_task(async function runTestAuth() {
-  // Following tests chack prompt message for a cross-origin and not
+  // Following tests check prompt message for a cross-origin and not
   // cross-origin subresources load
 
-
-  // Auth prompts are window modal
-  modalType = Ci.nsIPrompt.MODAL_TYPE_WINDOW;
+  // Let prompt_common know what kind of modal type is enabled for auth prompts.
+  modalType = authPromptModalType;
 
   let state, action;
 
diff --git a/toolkit/mozapps/extensions/test/xpinstall/head.js b/toolkit/mozapps/extensions/test/xpinstall/head.js
index 3c09face734c..90b1853c709e 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/head.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/head.js
@@ -3,6 +3,9 @@
 const { PermissionTestUtils } = ChromeUtils.import(
   "resource://testing-common/PermissionTestUtils.jsm"
 );
+const { PromptTestUtils } = ChromeUtils.import(
+  "resource://testing-common/PromptTestUtils.jsm"
+);
 
 const RELATIVE_DIR = "toolkit/mozapps/extensions/test/xpinstall/";
 
@@ -118,12 +121,14 @@ var Harness = {
       Services.obs.addObserver(this, "addon-install-failed");
       Services.obs.addObserver(this, "addon-install-complete");
 
+      // For browser_auth tests which trigger auth dialogs.
+      Services.obs.addObserver(this, "tabmodal-dialog-loaded");
+      Services.obs.addObserver(this, "common-dialog-loaded");
+
       this._boundWin = Cu.getWeakReference(win); // need this so our addon manager listener knows which window to use.
       AddonManager.addInstallListener(this);
       AddonManager.addAddonListener(this);
 
-      Services.wm.addListener(this);
-
       win.addEventListener("popupshown", this);
       win.PanelUI.notificationPanel.addEventListener("popupshown", this);
 
@@ -143,11 +148,12 @@ var Harness = {
         Services.obs.removeObserver(self, "addon-install-failed");
         Services.obs.removeObserver(self, "addon-install-complete");
 
+        Services.obs.removeObserver(self, "tabmodal-dialog-loaded");
+        Services.obs.removeObserver(self, "common-dialog-loaded");
+
         AddonManager.removeInstallListener(self);
         AddonManager.removeAddonListener(self);
 
-        Services.wm.removeListener(self);
-
         win.removeEventListener("popupshown", self);
         win.PanelUI.notificationPanel.removeEventListener("popupshown", self);
         win = null;
@@ -223,40 +229,38 @@ var Harness = {
     }
   },
 
-  // Window open handling
-  windowReady(window) {
-    if (window.document.location.href == PROMPT_URL) {
-      var promptType = window.args.promptType;
-      let dialog = window.document.getElementById("commonDialog");
-      switch (promptType) {
-        case "alert":
-        case "alertCheck":
-        case "confirmCheck":
-        case "confirm":
-        case "confirmEx":
-          dialog.acceptDialog();
-          break;
-        case "promptUserAndPass":
-          // This is a login dialog, hopefully an authentication prompt
-          // for the xpi.
-          if (this.authenticationCallback) {
-            var auth = this.authenticationCallback();
-            if (auth && auth.length == 2) {
-              window.document.getElementById("loginTextbox").value = auth[0];
-              window.document.getElementById("password1Textbox").value =
-                auth[1];
-              dialog.acceptDialog();
-            } else {
-              dialog.cancelDialog();
-            }
+  promptReady(dialog) {
+    let promptType = dialog.args.promptType;
+
+    switch (promptType) {
+      case "alert":
+      case "alertCheck":
+      case "confirmCheck":
+      case "confirm":
+      case "confirmEx":
+        PromptTestUtils.handlePrompt(dialog, { buttonNumClick: 0 });
+        break;
+      case "promptUserAndPass":
+        // This is a login dialog, hopefully an authentication prompt
+        // for the xpi.
+        if (this.authenticationCallback) {
+          var auth = this.authenticationCallback();
+          if (auth && auth.length == 2) {
+            PromptTestUtils.handlePrompt(dialog, {
+              loginInput: auth[0],
+              passwordInput: auth[1],
+              buttonNumClick: 0,
+            });
           } else {
-            dialog.cancelDialog();
+            PromptTestUtils.handlePrompt(dialog, { buttonNumClick: 1 });
           }
-          break;
-        default:
-          ok(false, "prompt type " + promptType + " not handled in test.");
-          break;
-      }
+        } else {
+          PromptTestUtils.handlePrompt(dialog, { buttonNumClick: 1 });
+        }
+        break;
+      default:
+        ok(false, "prompt type " + promptType + " not handled in test.");
+        break;
     }
   },
 
@@ -357,18 +361,6 @@ var Harness = {
     }
   },
 
-  // nsIWindowMediatorListener
-
-  onOpenWindow(xulWin) {
-    var domwindow = xulWin.docShell.domWindow;
-    var self = this;
-    waitForFocus(function() {
-      self.windowReady(domwindow);
-    }, domwindow);
-  },
-
-  onCloseWindow(window) {},
-
   // Addon Install Listener
 
   onNewInstall(install) {
@@ -556,11 +548,16 @@ var Harness = {
           );
         }, this);
         break;
+      case "tabmodal-dialog-loaded":
+        let browser = subject.ownerGlobal.gBrowser.selectedBrowser;
+        let prompt = browser.tabModalPromptBox.getPrompt(subject);
+        this.promptReady(prompt.Dialog);
+        break;
+      case "common-dialog-loaded":
+        this.promptReady(subject.Dialog);
+        break;
     }
   },
 
-  QueryInterface: ChromeUtils.generateQI([
-    "nsIObserver",
-    "nsIWindowMediatorListener",
-  ]),
+  QueryInterface: ChromeUtils.generateQI(["nsIObserver"]),
 };