--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -12,17 +12,16 @@
 #include "nsString.h"
 #include "nsReadableUtils.h"
 #include "nsIContent.h"
 #include "nsIContentViewerContainer.h"
 #include "nsIContentViewer.h"
 #include "nsIDocumentViewerPrint.h"
 #include "nsIDOMBeforeUnloadEvent.h"
 #include "nsIDocument.h"
-#include "nsIDOMWindowUtils.h"
 #include "nsPresContext.h"
 #include "nsIPresShell.h"
 #include "nsStyleSet.h"
 #include "nsIFrame.h"
 #include "nsIWritablePropertyBag2.h"
 #include "nsSubDocumentFrame.h"
 
 #include "nsILinkHandler.h"
@@ -64,16 +63,17 @@
 #ifdef MOZ_XUL
 #include "nsIXULDocument.h"
 #include "nsXULPopupManager.h"
 #endif
 
 #include "nsIClipboardHelper.h"
 
 #include "nsPIDOMWindow.h"
+#include "nsGlobalWindow.h"
 #include "nsDOMNavigationTiming.h"
 #include "nsPIWindowRoot.h"
 #include "nsJSEnvironment.h"
 #include "nsFocusManager.h"
 
 #include "nsIScrollableFrame.h"
 #include "nsStyleSheetService.h"
 #include "nsRenderingContext.h"
@@ -1110,41 +1110,41 @@ nsDocumentViewer::PermitUnloadInternal(b
   // Dispatching to |window|, but using |document| as the target.
   event->SetTarget(mDocument);
   event->SetTrusted(true);
 
   // In evil cases we might be destroyed while handling the
   // onbeforeunload event, don't let that happen. (see also bug#331040)
   nsRefPtr<nsDocumentViewer> kungFuDeathGrip(this);
 
+  bool dialogsAreEnabled = false;
   {
     // Never permit popups from the beforeunload handler, no matter
     // how we get here.
     nsAutoPopupStatePusher popupStatePusher(openAbused, true);
 
     // Never permit dialogs from the beforeunload handler
-    nsCOMPtr<nsIDOMWindowUtils> utils = do_GetInterface(window);
-    bool dialogsWereEnabled = false;
-    utils->AreDialogsEnabled(&dialogsWereEnabled);
-    utils->DisableDialogs();
+    nsGlobalWindow *globalWindow = static_cast<nsGlobalWindow*>(window);
+    dialogsAreEnabled = globalWindow->AreDialogsEnabled();
+    globalWindow->DisableDialogs();
 
     mInPermitUnload = true;
     EventDispatcher::DispatchDOMEvent(window, nullptr, event, mPresContext,
                                       nullptr);
     mInPermitUnload = false;
-    if (dialogsWereEnabled) {
-      utils->EnableDialogs();
+    if (dialogsAreEnabled) {
+      globalWindow->EnableDialogs();
     }
   }
 
   nsCOMPtr<nsIDocShell> docShell(mContainer);
   nsAutoString text;
   beforeUnload->GetReturnValue(text);
 
-  if (!sIsBeforeUnloadDisabled && *aShouldPrompt &&
+  if (!sIsBeforeUnloadDisabled && *aShouldPrompt && dialogsAreEnabled &&
       (event->GetInternalNSEvent()->mFlags.mDefaultPrevented ||
        !text.IsEmpty())) {
     // Ask the user if it's ok to unload the current page
 
     nsCOMPtr<nsIPrompt> prompt = do_GetInterface(docShell);
 
     if (prompt) {
       nsCOMPtr<nsIWritablePropertyBag2> promptBag = do_QueryInterface(prompt);

--- a/layout/base/tests/browser.ini
+++ b/layout/base/tests/browser.ini
@@ -1,4 +1,5 @@
 [DEFAULT]
 
 [browser_bug617076.js]
 skip-if = e10s # Bug ?????? - test touches content (TypeError: doc.documentElement is null)
+[browser_disableDialogs_onbeforeunload.js]

new file mode 100644
--- /dev/null
+++ b/layout/base/tests/browser_disableDialogs_onbeforeunload.js
@@ -0,0 +1,54 @@
+function pageScript() {
+  window.addEventListener("beforeunload", function (event) {
+    var str = "Some text that causes the beforeunload dialog to be shown";
+    event.returnValue = str;
+    return str;
+  }, true);
+}
+
+const PAGE_URL =
+  "data:text/html," + encodeURIComponent("<script>(" + pageScript.toSource() + ")();</script>");
+
+add_task(function* enableDialogs() {
+  // The onbeforeunload dialog should appear.
+  let dialogShown = false;
+  function onDialogShown(node) {
+    dialogShown = true;
+    let dismissButton = node.ui.button0;
+    dismissButton.click();
+  }
+  let obsName = "tabmodal-dialog-loaded";
+  Services.obs.addObserver(onDialogShown, obsName, false);
+  yield openPage(true);
+  Services.obs.removeObserver(onDialogShown, obsName);
+  Assert.ok(dialogShown);
+});
+
+add_task(function* disableDialogs() {
+  // The onbeforeunload dialog should NOT appear.
+  yield openPage(false);
+  info("If we time out here, then the dialog was shown...");
+});
+
+function* openPage(enableDialogs) {
+  // Open about:blank in a new tab.
+  yield BrowserTestUtils.withNewTab({ gBrowser, url: "about:blank" }, function* (browser) {
+    // Load the content script in the frame.
+    let methodName = enableDialogs ? "enableDialogs" : "disableDialogs";
+    yield ContentTask.spawn(browser, methodName, function* (name) {
+      Components.utils.import("resource://gre/modules/Services.jsm");
+      Services.obs.addObserver(doc => {
+        if (content && doc == content.document) {
+          content.QueryInterface(Ci.nsIInterfaceRequestor).
+            getInterface(Ci.nsIDOMWindowUtils)[name]();
+        }
+      }, "document-element-inserted", false);
+    });
+    // Load the page.
+    yield BrowserTestUtils.loadURI(browser, PAGE_URL);
+    yield BrowserTestUtils.browserLoaded(browser);
+    // And then navigate away.
+    yield BrowserTestUtils.loadURI(browser, "http://example.com/");
+    yield BrowserTestUtils.browserLoaded(browser);
+  });
+}