--- a/dom/base/nsContentPermissionHelper.cpp
+++ b/dom/base/nsContentPermissionHelper.cpp
@@ -641,24 +641,24 @@ nsContentPermissionRequestProxy::GetPrin
     return NS_ERROR_FAILURE;
   }
 
   NS_ADDREF(*aRequestingPrincipal = mParent->mPrincipal);
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsContentPermissionRequestProxy::GetElement(nsIDOMElement * *aRequestingElement)
+nsContentPermissionRequestProxy::GetElement(Element** aRequestingElement)
 {
   NS_ENSURE_ARG_POINTER(aRequestingElement);
   if (mParent == nullptr) {
     return NS_ERROR_FAILURE;
   }
 
-  nsCOMPtr<nsIDOMElement> elem = do_QueryInterface(mParent->mElement);
+  nsCOMPtr<Element> elem = mParent->mElement;
   elem.forget(aRequestingElement);
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsContentPermissionRequestProxy::GetIsHandlingUserInput(bool* aIsHandlingUserInput)
 {
   NS_ENSURE_ARG_POINTER(aIsHandlingUserInput);

--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -2045,28 +2045,16 @@ nsContentUtils::Shutdown()
  * Checks whether two nodes come from the same origin. aTrustedNode is
  * considered 'safe' in that a user can operate on it and that it isn't
  * a js-object that implements nsIDOMNode.
  * Never call this function with the first node provided by script, it
  * must always be known to be a 'real' node!
  */
 // static
 nsresult
-nsContentUtils::CheckSameOrigin(const nsINode *aTrustedNode,
-                                nsIDOMNode *aUnTrustedNode)
-{
-  MOZ_ASSERT(aTrustedNode);
-
-  // Make sure it's a real node.
-  nsCOMPtr<nsINode> unTrustedNode = do_QueryInterface(aUnTrustedNode);
-  NS_ENSURE_TRUE(unTrustedNode, NS_ERROR_UNEXPECTED);
-  return CheckSameOrigin(aTrustedNode, unTrustedNode);
-}
-
-nsresult
 nsContentUtils::CheckSameOrigin(const nsINode* aTrustedNode,
                                 const nsINode* unTrustedNode)
 {
   MOZ_ASSERT(aTrustedNode);
   MOZ_ASSERT(unTrustedNode);
 
   /*
    * Get hold of each node's principal

--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -586,18 +586,16 @@ public:
   static int32_t ParseLegacyFontSize(const nsAString& aValue);
 
   static void Shutdown();
 
   /**
    * Checks whether two nodes come from the same origin.
    */
   static nsresult CheckSameOrigin(const nsINode* aTrustedNode,
-                                  nsIDOMNode* aUnTrustedNode);
-  static nsresult CheckSameOrigin(const nsINode* aTrustedNode,
                                   const nsINode* unTrustedNode);
 
   // Check if the (JS) caller can access aNode.
   static bool CanCallerAccess(nsIDOMNode *aNode);
   static bool CanCallerAccess(nsINode* aNode);
 
   // Check if the (JS) caller can access aWindow.
   // aWindow can be either outer or inner window.

--- a/dom/base/nsIObjectLoadingContent.idl
+++ b/dom/base/nsIObjectLoadingContent.idl
@@ -4,17 +4,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsISupports.idl"
 
 interface nsIRequest;
 interface nsIFrame;
 interface nsIObjectFrame;
 interface nsIPluginTag;
-interface nsIDOMElement;
 interface nsIURI;
 
 %{C++
 class nsNPAPIPluginInstance;
 %}
 [ptr] native nsNPAPIPluginInstancePtr(nsNPAPIPluginInstance);
 
 /**

--- a/dom/geolocation/nsGeolocation.cpp
+++ b/dom/geolocation/nsGeolocation.cpp
@@ -391,17 +391,17 @@ nsGeolocationRequest::GetWindow(mozIDOMW
 
   nsCOMPtr<nsPIDOMWindowInner> window = do_QueryReferent(mLocator->GetOwner());
   window.forget(aRequestingWindow);
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsGeolocationRequest::GetElement(nsIDOMElement * *aRequestingElement)
+nsGeolocationRequest::GetElement(Element** aRequestingElement)
 {
   NS_ENSURE_ARG_POINTER(aRequestingElement);
   *aRequestingElement = nullptr;
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsGeolocationRequest::GetIsHandlingUserInput(bool* aIsHandlingUserInput)

--- a/dom/html/nsIMenuBuilder.idl
+++ b/dom/html/nsIMenuBuilder.idl
@@ -1,16 +1,16 @@
 /* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* 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/. */
 
 #include "nsISupports.idl"
 
-interface nsIDOMElement;
+webidl Element;
 
 /**
  * An interface used to construct native toolbar or context menus from <menu>
  */
 
 [scriptable, uuid(93F4A48F-D043-4F45-97FD-9771EA1AF976)]
 interface nsIMenuBuilder : nsISupports
 {
@@ -23,17 +23,17 @@ interface nsIMenuBuilder : nsISupports
   void openContainer(in DOMString aLabel);
 
   /**
    * Add a new menu item. All menu item details can be obtained from
    * the element. This method is not called for hidden elements or elements
    * with no or empty label. The icon should be loaded only if aCanLoadIcon
    * is true.
    */
-  void addItemFor(in nsIDOMElement aElement,
+  void addItemFor(in Element aElement,
                   in boolean aCanLoadIcon);
 
   /**
    * Create a new separator.
    */
   void addSeparator();
 
   /**

--- a/dom/interfaces/base/nsIContentPermissionPrompt.idl
+++ b/dom/interfaces/base/nsIContentPermissionPrompt.idl
@@ -1,19 +1,20 @@
 /* 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/. */
 
 #include "nsISupports.idl"
 
 interface nsIPrincipal;
 interface mozIDOMWindow;
-interface nsIDOMElement;
 interface nsIArray;
 
+webidl Element;
+
 /**
  *  Interface provides the request type and its access.
  */
 [scriptable, uuid(ef4db3b8-ca9c-4b1d-8f81-fd88ec32af13)]
 interface nsIContentPermissionType : nsISupports {
   /**
    *  The type of the permission request, such as
    *  "geolocation".
@@ -80,17 +81,17 @@ interface nsIContentPermissionRequest : 
 
   /**
    *  The window or element that the permission request was
    *  originated in.  Typically the element will be non-null
    *  in when using out of process content.  window or
    *  element can be null but not both.
    */
   readonly attribute mozIDOMWindow window;
-  readonly attribute nsIDOMElement element;
+  readonly attribute Element element;
 
   readonly attribute boolean isHandlingUserInput;
 
   /**
    *  The requester to get the required information of
    *  the window.
    */
   readonly attribute nsIContentPermissionRequester requester;

--- a/dom/interfaces/base/nsIContentProcess.idl
+++ b/dom/interfaces/base/nsIContentProcess.idl
@@ -1,15 +1,14 @@
 /* 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/. */
 
 #include "nsISupports.idl"
 
-interface nsIDOMElement;
 interface nsIURI;
 
 [scriptable, builtinclass, uuid(456f58be-29dd-4973-885b-95aece1c9a8a)]
 interface nsIContentProcessInfo : nsISupports
 {
   /**
    * Is this content process alive?
    */

--- a/dom/interfaces/base/nsIDOMChromeWindow.idl
+++ b/dom/interfaces/base/nsIDOMChromeWindow.idl
@@ -1,17 +1,16 @@
 /* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* 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/. */
 
 #include "domstubs.idl"
 
 interface nsIBrowserDOMWindow;
-interface nsIDOMElement;
 interface mozIDOMWindowProxy;
 
 // Scriptable only so Components.interfaces.nsIDOMChromeWindow works.
 [scriptable, builtinclass, uuid(78bdcb41-1efa-409f-aaba-70842213f80f)]
 interface nsIDOMChromeWindow : nsISupports
 {
   /**
    * browserDOMWindow provides access to yet another layer of

--- a/dom/interfaces/xul/nsIDOMXULCheckboxElement.idl
+++ b/dom/interfaces/xul/nsIDOMXULCheckboxElement.idl
@@ -1,12 +1,11 @@
 /* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* 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/. */
 
-#include "nsIDOMElement.idl"
 #include "nsIDOMXULLabeledControlEl.idl"
 
 [scriptable, uuid(674965d9-aff4-411d-a382-7cb32c0f25a1)]
 interface nsIDOMXULCheckboxElement : nsIDOMXULLabeledControlElement {
   attribute boolean checked;
 };

--- a/dom/interfaces/xul/nsIDOMXULCommandDispatcher.idl
+++ b/dom/interfaces/xul/nsIDOMXULCommandDispatcher.idl
@@ -1,32 +1,31 @@
 /* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* 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/. */
 
 #include "nsISupports.idl"
 
-interface nsIDOMElement;
 interface nsIController;
 interface nsIControllers;
 interface mozIDOMWindowProxy;
 
 webidl Element;
 
 [scriptable, uuid(a9fa9fd3-8d62-4f94-9ed8-3ea9c3cf0773)]
 interface nsIDOMXULCommandDispatcher : nsISupports
 {
             attribute Element focusedElement;
             attribute mozIDOMWindowProxy focusedWindow;
 
-  void                      addCommandUpdater(in nsIDOMElement updater,
+  void                      addCommandUpdater(in Element updater,
                                               in DOMString events,
                                               in DOMString targets);
-  void                      removeCommandUpdater(in nsIDOMElement updater);
+  void                      removeCommandUpdater(in Element updater);
 
   void                      updateCommands(in DOMString eventName);
 
   nsIController             getControllerForCommand(in string command);
   nsIControllers            getControllers();
 
   void advanceFocus();
   void rewindFocus();

--- a/dom/interfaces/xul/nsIDOMXULLabeledControlEl.idl
+++ b/dom/interfaces/xul/nsIDOMXULLabeledControlEl.idl
@@ -1,14 +1,13 @@
 /* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* 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/. */
 
-#include "nsIDOMElement.idl"
 #include "nsIDOMXULControlElement.idl"
 
 [scriptable, uuid(c58a159f-e27d-40c8-865a-d4dcfd928f62)]
 interface nsIDOMXULLabeledControlElement : nsIDOMXULControlElement {
   attribute DOMString crop;
   attribute DOMString image;
   attribute DOMString label;
   attribute DOMString accessKey;

--- a/dom/interfaces/xul/nsIDOMXULTextboxElement.idl
+++ b/dom/interfaces/xul/nsIDOMXULTextboxElement.idl
@@ -1,15 +1,16 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 /* 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/. */
 
 #include "nsIDOMXULLabeledControlEl.idl"
-interface nsIDOMHTMLInputElement;
+
+interface nsIDOMNode;
 
 [scriptable, uuid(7edd8215-5155-4845-a02f-dc2c08645cb9)]
 interface nsIDOMXULTextBoxElement : nsIDOMXULControlElement
 {
   // inputField may be any type of editable field, such as an
   // HTML <input type="text"> or <textarea>
   readonly attribute nsIDOMNode inputField;
 

--- a/dom/ipc/ProcessHangMonitor.cpp
+++ b/dom/ipc/ProcessHangMonitor.cpp
@@ -165,31 +165,17 @@ class HangMonitoredProcess final
 {
 public:
   NS_DECL_THREADSAFE_ISUPPORTS
 
   HangMonitoredProcess(HangMonitorParent* aActor,
                        ContentParent* aContentParent)
     : mActor(aActor), mContentParent(aContentParent) {}
 
-  NS_IMETHOD GetHangType(uint32_t* aHangType) override;
-  NS_IMETHOD GetScriptBrowser(nsIDOMElement** aBrowser) override;
-  NS_IMETHOD GetScriptFileName(nsACString& aFileName) override;
-  NS_IMETHOD GetAddonId(nsAString& aAddonId) override;
-
-  NS_IMETHOD GetPluginName(nsACString& aPluginName) override;
-
-  NS_IMETHOD TerminateScript() override;
-  NS_IMETHOD TerminateGlobal() override;
-  NS_IMETHOD BeginStartingDebugger() override;
-  NS_IMETHOD EndStartingDebugger() override;
-  NS_IMETHOD TerminatePlugin() override;
-  NS_IMETHOD UserCanceled() override;
-
-  NS_IMETHOD IsReportForBrowser(nsFrameLoader* aFrameLoader, bool* aResult) override;
+  NS_DECL_NSIHANGREPORT
 
   // Called when a content process shuts down.
   void Clear() {
     mContentParent = nullptr;
     mActor = nullptr;
   }
 
   /**
@@ -943,34 +929,34 @@ HangMonitoredProcess::GetHangType(uint32
     MOZ_ASSERT_UNREACHABLE("Unexpected HangData type");
     return NS_ERROR_UNEXPECTED;
   }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
-HangMonitoredProcess::GetScriptBrowser(nsIDOMElement** aBrowser)
+HangMonitoredProcess::GetScriptBrowser(Element** aBrowser)
 {
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
   if (mHangData.type() != HangData::TSlowScriptData) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
   TabId tabId = mHangData.get_SlowScriptData().tabId();
   if (!mContentParent) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
   nsTArray<PBrowserParent*> tabs;
   mContentParent->ManagedPBrowserParent(tabs);
   for (size_t i = 0; i < tabs.Length(); i++) {
     TabParent* tp = TabParent::GetFrom(tabs[i]);
     if (tp->GetTabId() == tabId) {
-      nsCOMPtr<nsIDOMElement> node = do_QueryInterface(tp->GetOwnerElement());
+      RefPtr<Element> node = tp->GetOwnerElement();
       node.forget(aBrowser);
       return NS_OK;
     }
   }
 
   *aBrowser = nullptr;
   return NS_OK;
 }

--- a/dom/ipc/nsIHangReport.idl
+++ b/dom/ipc/nsIHangReport.idl
@@ -1,19 +1,18 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* 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/. */
 
 #include "nsISupports.idl"
 
-interface nsIDOMElement;
-
 webidl FrameLoader;
+webidl Element;
 
 /**
  * When a content process hangs, Gecko notifies "process-hang-report" observers
  * and passes an nsIHangReport for the subject parameter. There is at most one
  * nsIHangReport associated with a given content process. As long as the content
  * process stays stuck, the "process-hang-report" observer will continue to be
  * notified at regular intervals (approximately once per second). The content
  * process will continue to run uninhibitedly during this time.
@@ -26,17 +25,17 @@ interface nsIHangReport : nsISupports
   const unsigned long PLUGIN_HANG = 2;
 
   // The type of hang being reported: SLOW_SCRIPT or PLUGIN_HANG.
   readonly attribute unsigned long hangType;
 
   // For SLOW_SCRIPT reports, these fields contain information about the
   // slow script.
   // Only valid for SLOW_SCRIPT reports.
-  readonly attribute nsIDOMElement scriptBrowser;
+  readonly attribute Element scriptBrowser;
   readonly attribute ACString scriptFileName;
   readonly attribute AString addonId;
 
   // For PLUGIN_HANGs, this field contains information about the plugin.
   // Only valid for PLUGIN_HANG reports.
   readonly attribute ACString pluginName;
 
   // Called by front end code when user ignores or cancels

--- a/dom/midi/MIDIPermissionRequest.cpp
+++ b/dom/midi/MIDIPermissionRequest.cpp
@@ -86,17 +86,17 @@ NS_IMETHODIMP
 MIDIPermissionRequest::GetWindow(mozIDOMWindow** aRequestingWindow)
 {
   NS_ENSURE_ARG_POINTER(aRequestingWindow);
   NS_IF_ADDREF(*aRequestingWindow = mWindow);
   return NS_OK;
 }
 
 NS_IMETHODIMP
-MIDIPermissionRequest::GetElement(nsIDOMElement** aRequestingElement)
+MIDIPermissionRequest::GetElement(Element** aRequestingElement)
 {
   NS_ENSURE_ARG_POINTER(aRequestingElement);
   *aRequestingElement = nullptr;
   return NS_OK;
 }
 
 NS_IMETHODIMP
 MIDIPermissionRequest::Cancel()

--- a/dom/notification/Notification.cpp
+++ b/dom/notification/Notification.cpp
@@ -589,17 +589,17 @@ NotificationPermissionRequest::GetPrinci
 NS_IMETHODIMP
 NotificationPermissionRequest::GetWindow(mozIDOMWindow** aRequestingWindow)
 {
   NS_ADDREF(*aRequestingWindow = mWindow);
   return NS_OK;
 }
 
 NS_IMETHODIMP
-NotificationPermissionRequest::GetElement(nsIDOMElement** aElement)
+NotificationPermissionRequest::GetElement(Element** aElement)
 {
   NS_ENSURE_ARG_POINTER(aElement);
   *aElement = nullptr;
   return NS_OK;
 }
 
 NS_IMETHODIMP
 NotificationPermissionRequest::GetIsHandlingUserInput(bool* aIsHandlingUserInput)

--- a/dom/quota/StorageManager.cpp
+++ b/dom/quota/StorageManager.cpp
@@ -737,17 +737,17 @@ PersistentStoragePermissionRequest::GetW
   MOZ_ASSERT(mWindow);
 
   NS_ADDREF(*aRequestingWindow = mWindow);
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
-PersistentStoragePermissionRequest::GetElement(nsIDOMElement** aElement)
+PersistentStoragePermissionRequest::GetElement(Element** aElement)
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(aElement);
 
   *aElement = nullptr;
   return NS_OK;
 }
 

--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -1522,22 +1522,17 @@ XULDocument::RemoveSubtreeFromDocument(n
         nsAutoScriptBlocker scriptBlocker;
         RemoveFromIdTable(aElement, id);
     }
 
     // 3. If the element is a 'command updater', then remove the
     // element from the document's command dispatcher.
     if (aElement->AttrValueIs(kNameSpaceID_None, nsGkAtoms::commandupdater,
                               nsGkAtoms::_true, eCaseMatters)) {
-        nsCOMPtr<nsIDOMElement> domelement = do_QueryInterface(aElement);
-        NS_ASSERTION(domelement != nullptr, "not a DOM element");
-        if (! domelement)
-            return NS_ERROR_UNEXPECTED;
-
-        rv = mCommandDispatcher->RemoveCommandUpdater(domelement);
+        rv = mCommandDispatcher->RemoveCommandUpdater(aElement);
         if (NS_FAILED(rv)) return rv;
     }
 
     // 4. Remove the element from our broadcaster map, since it is no longer
     // in the document.
     nsCOMPtr<Element> broadcaster, listener;
     nsAutoString attribute, broadcasterID;
     rv = FindBroadcaster(aElement, getter_AddRefs(listener),

--- a/dom/xul/nsXULCommandDispatcher.cpp
+++ b/dom/xul/nsXULCommandDispatcher.cpp
@@ -235,17 +235,17 @@ nsXULCommandDispatcher::AdvanceFocusInto
   nsIFocusManager* fm = nsFocusManager::GetFocusManager();
   if (fm)
     return fm->MoveFocus(win, aElt, nsIFocusManager::MOVEFOCUS_FORWARD,
                          0, getter_AddRefs(result));
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsXULCommandDispatcher::AddCommandUpdater(nsIDOMElement* aElement,
+nsXULCommandDispatcher::AddCommandUpdater(Element* aElement,
                                           const nsAString& aEvents,
                                           const nsAString& aTargets)
 {
   NS_PRECONDITION(aElement != nullptr, "null ptr");
   if (! aElement)
     return NS_ERROR_NULL_POINTER;
 
   NS_ENSURE_TRUE(mDocument, NS_ERROR_UNEXPECTED);
@@ -305,17 +305,17 @@ nsXULCommandDispatcher::AddCommandUpdate
 #endif
 
   // If we get here, this is a new updater. Append it to the list.
   *link = new Updater(aElement, aEvents, aTargets);
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsXULCommandDispatcher::RemoveCommandUpdater(nsIDOMElement* aElement)
+nsXULCommandDispatcher::RemoveCommandUpdater(Element* aElement)
 {
   NS_PRECONDITION(aElement != nullptr, "null ptr");
   if (! aElement)
     return NS_ERROR_NULL_POINTER;
 
   Updater* updater = mUpdaters;
   Updater** link = &mUpdaters;
 

--- a/dom/xul/nsXULCommandDispatcher.h
+++ b/dom/xul/nsXULCommandDispatcher.h
@@ -14,18 +14,18 @@
 #define nsXULCommandDispatcher_h__
 
 #include "nsCOMPtr.h"
 #include "nsIDOMXULCommandDispatcher.h"
 #include "nsWeakReference.h"
 #include "nsIDOMNode.h"
 #include "nsString.h"
 #include "nsCycleCollectionParticipant.h"
+#include "mozilla/RefPtr.h"
 
-class nsIDOMElement;
 class nsPIWindowRoot;
 
 namespace mozilla {
 namespace dom {
 class Element;
 } // namespace dom
 } // namespace mozilla
 
@@ -51,26 +51,26 @@ protected:
 
     mozilla::dom::Element*
       GetRootFocusedContentAndWindow(nsPIDOMWindowOuter** aWindow);
 
     nsCOMPtr<nsIDocument> mDocument;
 
     class Updater {
     public:
-      Updater(nsIDOMElement* aElement,
+      Updater(mozilla::dom::Element* aElement,
               const nsAString& aEvents,
               const nsAString& aTargets)
           : mElement(aElement),
             mEvents(aEvents),
             mTargets(aTargets),
             mNext(nullptr)
       {}
 
-      nsCOMPtr<nsIDOMElement> mElement;
+      RefPtr<mozilla::dom::Element> mElement;
       nsString                mEvents;
       nsString                mTargets;
       Updater*                mNext;
     };
 
     Updater* mUpdaters;
 
     bool Matches(const nsString& aList,

--- a/dom/xul/nsXULContentUtils.cpp
+++ b/dom/xul/nsXULContentUtils.cpp
@@ -138,19 +138,14 @@ nsXULContentUtils::SetCommandUpdater(nsI
         events.Assign('*');
 
     nsAutoString targets;
     aElement->GetAttr(kNameSpaceID_None, nsGkAtoms::targets, targets);
 
     if (targets.IsEmpty())
         targets.Assign('*');
 
-    nsCOMPtr<nsIDOMElement> domelement = do_QueryInterface(aElement);
-    NS_ASSERTION(domelement != nullptr, "not a DOM element");
-    if (! domelement)
-        return NS_ERROR_UNEXPECTED;
-
-    rv = dispatcher->AddCommandUpdater(domelement, events, targets);
+    rv = dispatcher->AddCommandUpdater(aElement, events, targets);
     if (NS_FAILED(rv)) return rv;
 
     return NS_OK;
 }