Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

98 files changed, 13855 insertions, 0 deletions

diff --git a/dom/xul/ChromeObserver.cpp b/dom/xul/ChromeObserver.cpp
new file mode 100644
index 0000000000..9ad66348d4
--- /dev/null
+++ b/dom/xul/ChromeObserver.cpp
@@ -0,0 +1,197 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+#include "ChromeObserver.h"
+
+#include "nsIBaseWindow.h"
+#include "nsIWidget.h"
+#include "nsIFrame.h"
+
+#include "nsContentUtils.h"
+#include "nsView.h"
+#include "nsPresContext.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/MutationEventBinding.h"
+#include "nsXULElement.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_ISUPPORTS(ChromeObserver, nsIMutationObserver)
+
+ChromeObserver::ChromeObserver(Document* aDocument)
+    : nsStubMutationObserver(), mDocument(aDocument) {}
+
+ChromeObserver::~ChromeObserver() = default;
+
+void ChromeObserver::Init() {
+  mDocument->AddMutationObserver(this);
+  Element* rootElement = mDocument->GetRootElement();
+  if (!rootElement) {
+    return;
+  }
+  nsAutoScriptBlocker scriptBlocker;
+  uint32_t attributeCount = rootElement->GetAttrCount();
+  for (uint32_t i = 0; i < attributeCount; i++) {
+    BorrowedAttrInfo info = rootElement->GetAttrInfoAt(i);
+    const nsAttrName* name = info.mName;
+    if (name->LocalName() == nsGkAtoms::chromemargin) {
+      // Some linux windows managers have an issue when the chrome margin is
+      // applied while the browser is loading (bug 1598848). For now, skip
+      // applying this attribute when initializing.
+      continue;
+    }
+    AttributeChanged(rootElement, name->NamespaceID(), name->LocalName(),
+                     MutationEvent_Binding::ADDITION, nullptr);
+  }
+}
+
+nsIWidget* ChromeObserver::GetWindowWidget() {
+  // only top level chrome documents can set the titlebar color
+  if (mDocument && mDocument->IsRootDisplayDocument()) {
+    nsCOMPtr<nsISupports> container = mDocument->GetContainer();
+    nsCOMPtr<nsIBaseWindow> baseWindow = do_QueryInterface(container);
+    if (baseWindow) {
+      nsCOMPtr<nsIWidget> mainWidget;
+      baseWindow->GetMainWidget(getter_AddRefs(mainWidget));
+      return mainWidget;
+    }
+  }
+  return nullptr;
+}
+
+void ChromeObserver::SetDrawsTitle(bool aState) {
+  nsIWidget* mainWidget = GetWindowWidget();
+  if (mainWidget) {
+    // We can do this synchronously because SetDrawsTitle doesn't have any
+    // synchronous effects apart from a harmless invalidation.
+    mainWidget->SetDrawsTitle(aState);
+  }
+}
+
+class MarginSetter : public Runnable {
+ public:
+  explicit MarginSetter(nsIWidget* aWidget)
+      : mozilla::Runnable("MarginSetter"),
+        mWidget(aWidget),
+        mMargin(-1, -1, -1, -1) {}
+  MarginSetter(nsIWidget* aWidget, const LayoutDeviceIntMargin& aMargin)
+      : mozilla::Runnable("MarginSetter"), mWidget(aWidget), mMargin(aMargin) {}
+
+  NS_IMETHOD Run() override {
+    // SetNonClientMargins can dispatch native events, hence doing
+    // it off a script runner.
+    mWidget->SetNonClientMargins(mMargin);
+    return NS_OK;
+  }
+
+ private:
+  nsCOMPtr<nsIWidget> mWidget;
+  LayoutDeviceIntMargin mMargin;
+};
+
+void ChromeObserver::SetChromeMargins(const nsAttrValue* aValue) {
+  if (!aValue) return;
+
+  nsIWidget* mainWidget = GetWindowWidget();
+  if (!mainWidget) return;
+
+  // top, right, bottom, left - see nsAttrValue
+  nsAutoString tmp;
+  aValue->ToString(tmp);
+  nsIntMargin margins;
+  if (nsContentUtils::ParseIntMarginValue(tmp, margins)) {
+    nsContentUtils::AddScriptRunner(new MarginSetter(
+        mainWidget, LayoutDeviceIntMargin::FromUnknownMargin(margins)));
+  }
+}
+
+void ChromeObserver::AttributeChanged(dom::Element* aElement,
+                                      int32_t aNamespaceID, nsAtom* aName,
+                                      int32_t aModType,
+                                      const nsAttrValue* aOldValue) {
+  // We only care about changes to the root element.
+  if (!mDocument || aElement != mDocument->GetRootElement()) {
+    return;
+  }
+
+  const nsAttrValue* value = aElement->GetParsedAttr(aName, aNamespaceID);
+  if (value) {
+    // Hide chrome if needed
+    if (aName == nsGkAtoms::hidechrome) {
+      HideWindowChrome(value->Equals(u"true"_ns, eCaseMatters));
+    } else if (aName == nsGkAtoms::chromemargin) {
+      SetChromeMargins(value);
+    }
+    // title and drawintitlebar are settable on
+    // any root node (windows, dialogs, etc)
+    else if (aName == nsGkAtoms::title) {
+      mDocument->NotifyPossibleTitleChange(false);
+    } else if (aName == nsGkAtoms::drawtitle) {
+      SetDrawsTitle(value->Equals(u"true"_ns, eCaseMatters));
+    } else if (aName == nsGkAtoms::localedir) {
+      // if the localedir changed on the root element, reset the document
+      // direction
+      mDocument->ResetDocumentDirection();
+    } else if (aName == nsGkAtoms::lwtheme) {
+      // if the lwtheme changed, make sure to reset the document lwtheme
+      // cache
+      mDocument->ResetDocumentLWTheme();
+    }
+  } else {
+    if (aName == nsGkAtoms::hidechrome) {
+      HideWindowChrome(false);
+    } else if (aName == nsGkAtoms::chromemargin) {
+      ResetChromeMargins();
+    } else if (aName == nsGkAtoms::localedir) {
+      // if the localedir changed on the root element, reset the document
+      // direction
+      mDocument->ResetDocumentDirection();
+    } else if (aName == nsGkAtoms::lwtheme) {
+      // if the lwtheme changed, make sure to restyle appropriately
+      mDocument->ResetDocumentLWTheme();
+    } else if (aName == nsGkAtoms::drawtitle) {
+      SetDrawsTitle(false);
+    }
+  }
+}
+
+void ChromeObserver::NodeWillBeDestroyed(nsINode* aNode) {
+  mDocument = nullptr;
+}
+
+void ChromeObserver::ResetChromeMargins() {
+  nsIWidget* mainWidget = GetWindowWidget();
+  if (!mainWidget) return;
+  // See nsIWidget
+  nsContentUtils::AddScriptRunner(new MarginSetter(mainWidget));
+}
+
+nsresult ChromeObserver::HideWindowChrome(bool aShouldHide) {
+  // only top level chrome documents can hide the window chrome
+  if (!mDocument->IsRootDisplayDocument()) return NS_OK;
+
+  nsPresContext* presContext = mDocument->GetPresContext();
+
+  if (presContext && presContext->IsChrome()) {
+    nsIFrame* frame = mDocument->GetDocumentElement()->GetPrimaryFrame();
+
+    if (frame) {
+      nsView* view = frame->GetClosestView();
+
+      if (view) {
+        nsIWidget* w = view->GetWidget();
+        NS_ENSURE_STATE(w);
+        w->HideWindowChrome(aShouldHide);
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/ChromeObserver.h b/dom/xul/ChromeObserver.h
new file mode 100644
index 0000000000..c651389487
--- /dev/null
+++ b/dom/xul/ChromeObserver.h
@@ -0,0 +1,42 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_dom_ChromeObserver_h
+#define mozilla_dom_ChromeObserver_h
+
+#include "nsStubMutationObserver.h"
+
+class nsIWidget;
+
+namespace mozilla::dom {
+class Document;
+
+class ChromeObserver final : public nsStubMutationObserver {
+ public:
+  NS_DECL_ISUPPORTS
+
+  explicit ChromeObserver(Document* aDocument);
+  void Init();
+
+  NS_DECL_NSIMUTATIONOBSERVER_ATTRIBUTECHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_NODEWILLBEDESTROYED
+
+ protected:
+  nsIWidget* GetWindowWidget();
+  void SetDrawsTitle(bool aState);
+  void SetChromeMargins(const nsAttrValue* aValue);
+  nsresult HideWindowChrome(bool aShouldHide);
+
+ private:
+  void ResetChromeMargins();
+  ~ChromeObserver();
+  // A weak pointer cleared when the element will be destroyed.
+  Document* MOZ_NON_OWNING_REF mDocument;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ChromeObserver_h
diff --git a/dom/xul/MenuBarListener.cpp b/dom/xul/MenuBarListener.cpp
new file mode 100644
index 0000000000..995ec422c5
--- /dev/null
+++ b/dom/xul/MenuBarListener.cpp
@@ -0,0 +1,489 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "MenuBarListener.h"
+#include "XULButtonElement.h"
+#include "mozilla/Attributes.h"
+#include "nsISound.h"
+
+// Drag & Drop, Clipboard
+#include "nsWidgetsCID.h"
+#include "nsCOMPtr.h"
+
+#include "nsContentUtils.h"
+#include "nsPIWindowRoot.h"
+#include "nsIFrame.h"
+#include "mozilla/BasicEvents.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/LookAndFeel.h"
+#include "mozilla/StaticPrefs_ui.h"
+#include "mozilla/TextEvents.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/EventBinding.h"
+#include "mozilla/dom/KeyboardEvent.h"
+#include "mozilla/dom/KeyboardEventBinding.h"
+#include "mozilla/dom/XULButtonElement.h"
+#include "mozilla/dom/XULMenuBarElement.h"
+#include "mozilla/dom/XULMenuParentElement.h"
+#include "nsXULPopupManager.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_ISUPPORTS(MenuBarListener, nsIDOMEventListener)
+
+MenuBarListener::MenuBarListener(XULMenuBarElement& aElement)
+    : mMenuBar(&aElement),
+      mEventTarget(aElement.GetComposedDoc()),
+      mAccessKeyDown(false),
+      mAccessKeyDownCanceled(false) {
+  MOZ_ASSERT(mEventTarget);
+  MOZ_ASSERT(mMenuBar);
+
+  // Hook up the menubar as a key listener on the whole document.  This will
+  // see every keypress that occurs, but after everyone else does.
+
+  // Also hook up the listener to the window listening for focus events. This
+  // is so we can keep proper state as the user alt-tabs through processes.
+
+  mEventTarget->AddSystemEventListener(u"keypress"_ns, this, false);
+  mEventTarget->AddSystemEventListener(u"keydown"_ns, this, false);
+  mEventTarget->AddSystemEventListener(u"keyup"_ns, this, false);
+  mEventTarget->AddSystemEventListener(u"mozaccesskeynotfound"_ns, this, false);
+  // Need a capturing event listener if the user has blocked pages from
+  // overriding system keys so that we can prevent menu accesskeys from being
+  // cancelled.
+  mEventTarget->AddEventListener(u"keydown"_ns, this, true);
+
+  // mousedown event should be handled in all phase
+  mEventTarget->AddEventListener(u"mousedown"_ns, this, true);
+  mEventTarget->AddEventListener(u"mousedown"_ns, this, false);
+  mEventTarget->AddEventListener(u"blur"_ns, this, true);
+
+  mEventTarget->AddEventListener(u"MozDOMFullscreen:Entered"_ns, this, false);
+
+  // Needs to listen to the deactivate event of the window.
+  RefPtr<EventTarget> top = nsContentUtils::GetWindowRoot(mEventTarget);
+  if (!NS_WARN_IF(!top)) {
+    top->AddSystemEventListener(u"deactivate"_ns, this, true);
+  }
+}
+
+////////////////////////////////////////////////////////////////////////
+MenuBarListener::~MenuBarListener() {
+  MOZ_ASSERT(!mEventTarget, "Should've detached always");
+}
+
+void MenuBarListener::Detach() {
+  if (!mMenuBar) {
+    MOZ_ASSERT(!mEventTarget);
+    return;
+  }
+  mEventTarget->RemoveSystemEventListener(u"keypress"_ns, this, false);
+  mEventTarget->RemoveSystemEventListener(u"keydown"_ns, this, false);
+  mEventTarget->RemoveSystemEventListener(u"keyup"_ns, this, false);
+  mEventTarget->RemoveSystemEventListener(u"mozaccesskeynotfound"_ns, this,
+                                          false);
+  mEventTarget->RemoveEventListener(u"keydown"_ns, this, true);
+
+  mEventTarget->RemoveEventListener(u"mousedown"_ns, this, true);
+  mEventTarget->RemoveEventListener(u"mousedown"_ns, this, false);
+  mEventTarget->RemoveEventListener(u"blur"_ns, this, true);
+
+  mEventTarget->RemoveEventListener(u"MozDOMFullscreen:Entered"_ns, this,
+                                    false);
+  RefPtr<EventTarget> top = nsContentUtils::GetWindowRoot(mEventTarget);
+  if (!NS_WARN_IF(!top)) {
+    top->RemoveSystemEventListener(u"deactivate"_ns, this, true);
+  }
+  mMenuBar = nullptr;
+  mEventTarget = nullptr;
+}
+
+void MenuBarListener::ToggleMenuActiveState(ByKeyboard aByKeyboard) {
+  RefPtr menuBar = mMenuBar;
+  if (menuBar->IsActive()) {
+    menuBar->SetActive(false);
+  } else {
+    if (aByKeyboard == ByKeyboard::Yes) {
+      menuBar->SetActiveByKeyboard();
+    }
+    // This will activate the menubar if needed.
+    menuBar->SelectFirstItem();
+  }
+}
+
+////////////////////////////////////////////////////////////////////////
+nsresult MenuBarListener::KeyUp(Event* aKeyEvent) {
+  WidgetKeyboardEvent* nativeKeyEvent =
+      aKeyEvent->WidgetEventPtr()->AsKeyboardEvent();
+  if (!nativeKeyEvent) {
+    return NS_OK;
+  }
+
+  // handlers shouldn't be triggered by non-trusted events.
+  if (!nativeKeyEvent->IsTrusted()) {
+    return NS_OK;
+  }
+
+  const auto accessKey = LookAndFeel::GetMenuAccessKey();
+  if (!accessKey || !StaticPrefs::ui_key_menuAccessKeyFocuses()) {
+    return NS_OK;
+  }
+
+  // On a press of the ALT key by itself, we toggle the menu's
+  // active/inactive state.
+  if (!nativeKeyEvent->DefaultPrevented() && mAccessKeyDown &&
+      !mAccessKeyDownCanceled && nativeKeyEvent->mKeyCode == accessKey) {
+    // The access key was down and is now up, and no other
+    // keys were pressed in between.
+    bool toggleMenuActiveState = true;
+    if (!mMenuBar->IsActive()) {
+      // If the focused content is in a remote process, we should allow the
+      // focused web app to prevent to activate the menubar.
+      if (nativeKeyEvent->WillBeSentToRemoteProcess()) {
+        nativeKeyEvent->StopImmediatePropagation();
+        nativeKeyEvent->MarkAsWaitingReplyFromRemoteProcess();
+        return NS_OK;
+      }
+      // First, close all existing popups because other popups shouldn't
+      // handle key events when menubar is active and IME should be
+      // disabled.
+      if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+        pm->Rollup({});
+      }
+      // If menubar active state is changed or the menubar is destroyed
+      // during closing the popups, we should do nothing anymore.
+      toggleMenuActiveState = !Destroyed() && !mMenuBar->IsActive();
+    }
+    if (toggleMenuActiveState) {
+      ToggleMenuActiveState(ByKeyboard::Yes);
+    }
+  }
+
+  mAccessKeyDown = false;
+  mAccessKeyDownCanceled = false;
+
+  if (!Destroyed() && mMenuBar->IsActive()) {
+    nativeKeyEvent->StopPropagation();
+    nativeKeyEvent->PreventDefault();
+  }
+
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////
+nsresult MenuBarListener::KeyPress(Event* aKeyEvent) {
+  // if event has already been handled, bail
+  if (!aKeyEvent || aKeyEvent->DefaultPrevented()) {
+    return NS_OK;  // don't consume event
+  }
+
+  // handlers shouldn't be triggered by non-trusted events.
+  if (!aKeyEvent->IsTrusted()) {
+    return NS_OK;
+  }
+
+  const auto accessKey = LookAndFeel::GetMenuAccessKey();
+  if (!accessKey) {
+    return NS_OK;
+  }
+  // If accesskey handling was forwarded to a child process, wait for
+  // the mozaccesskeynotfound event before handling accesskeys.
+  WidgetKeyboardEvent* nativeKeyEvent =
+      aKeyEvent->WidgetEventPtr()->AsKeyboardEvent();
+  if (!nativeKeyEvent) {
+    return NS_OK;
+  }
+
+  RefPtr<KeyboardEvent> keyEvent = aKeyEvent->AsKeyboardEvent();
+  uint32_t keyCode = keyEvent->KeyCode();
+
+  // Cancel the access key flag unless we are pressing the access key.
+  if (keyCode != accessKey) {
+    mAccessKeyDownCanceled = true;
+  }
+
+#ifndef XP_MACOSX
+  // Need to handle F10 specially on Non-Mac platform.
+  if (nativeKeyEvent->mMessage == eKeyPress && keyCode == NS_VK_F10) {
+    if ((keyEvent->GetModifiersForMenuAccessKey() & ~MODIFIER_CONTROL) == 0) {
+      // If the keyboard event should activate the menubar and will be
+      // sent to a remote process, it should be executed with reply
+      // event from the focused remote process.  Note that if the menubar
+      // is active, the event is already marked as "stop cross
+      // process dispatching".  So, in that case, this won't wait
+      // reply from the remote content.
+      if (nativeKeyEvent->WillBeSentToRemoteProcess()) {
+        nativeKeyEvent->StopImmediatePropagation();
+        nativeKeyEvent->MarkAsWaitingReplyFromRemoteProcess();
+        return NS_OK;
+      }
+      // The F10 key just went down by itself or with ctrl pressed.
+      // In Windows, both of these activate the menu bar.
+      ToggleMenuActiveState(ByKeyboard::Yes);
+
+      if (mMenuBar && mMenuBar->IsActive()) {
+#  ifdef MOZ_WIDGET_GTK
+        if (RefPtr child = mMenuBar->GetActiveMenuChild()) {
+          // In GTK, this also opens the first menu.
+          child->OpenMenuPopup(false);
+        }
+#  endif
+        aKeyEvent->StopPropagation();
+        aKeyEvent->PreventDefault();
+      }
+    }
+
+    return NS_OK;
+  }
+#endif  // !XP_MACOSX
+
+  RefPtr menuForKey = GetMenuForKeyEvent(*keyEvent);
+  if (!menuForKey) {
+#ifdef XP_WIN
+    // Behavior on Windows - this item is on the menu bar, beep and deactivate
+    // the menu bar.
+    // TODO(emilio): This is rather odd, and I cannot get the beep to work,
+    // but this matches what old code was doing...
+    if (mMenuBar && mMenuBar->IsActive() && mMenuBar->IsActiveByKeyboard()) {
+      if (nsCOMPtr<nsISound> sound = do_GetService("@mozilla.org/sound;1")) {
+        sound->Beep();
+      }
+      ToggleMenuActiveState(ByKeyboard::Yes);
+    }
+#endif
+    return NS_OK;
+  }
+
+  // If the keyboard event matches with a menu item's accesskey and
+  // will be sent to a remote process, it should be executed with
+  // reply event from the focused remote process.  Note that if the
+  // menubar is active, the event is already marked as "stop cross
+  // process dispatching".  So, in that case, this won't wait
+  // reply from the remote content.
+  if (nativeKeyEvent->WillBeSentToRemoteProcess()) {
+    nativeKeyEvent->StopImmediatePropagation();
+    nativeKeyEvent->MarkAsWaitingReplyFromRemoteProcess();
+    return NS_OK;
+  }
+
+  RefPtr menuBar = mMenuBar;
+  menuBar->SetActiveByKeyboard();
+  // This will activate the menubar as needed.
+  menuForKey->OpenMenuPopup(true);
+
+  // The opened menu will listen next keyup event.
+  // Therefore, we should clear the keydown flags here.
+  mAccessKeyDown = mAccessKeyDownCanceled = false;
+
+  aKeyEvent->StopPropagation();
+  aKeyEvent->PreventDefault();
+  return NS_OK;
+}
+
+dom::XULButtonElement* MenuBarListener::GetMenuForKeyEvent(
+    KeyboardEvent& aKeyEvent) {
+  if (!aKeyEvent.IsMenuAccessKeyPressed()) {
+    return nullptr;
+  }
+
+  uint32_t charCode = aKeyEvent.CharCode();
+  bool hasAccessKeyCandidates = charCode != 0;
+  if (!hasAccessKeyCandidates) {
+    WidgetKeyboardEvent* nativeKeyEvent =
+        aKeyEvent.WidgetEventPtr()->AsKeyboardEvent();
+    AutoTArray<uint32_t, 10> keys;
+    nativeKeyEvent->GetAccessKeyCandidates(keys);
+    hasAccessKeyCandidates = !keys.IsEmpty();
+  }
+
+  if (!hasAccessKeyCandidates) {
+    return nullptr;
+  }
+  // Do shortcut navigation.
+  // A letter was pressed. We want to see if a shortcut gets matched. If
+  // so, we'll know the menu got activated.
+  return mMenuBar->FindMenuWithShortcut(aKeyEvent);
+}
+
+void MenuBarListener::ReserveKeyIfNeeded(Event* aKeyEvent) {
+  WidgetKeyboardEvent* nativeKeyEvent =
+      aKeyEvent->WidgetEventPtr()->AsKeyboardEvent();
+  if (nsContentUtils::ShouldBlockReservedKeys(nativeKeyEvent)) {
+    nativeKeyEvent->MarkAsReservedByChrome();
+  }
+}
+
+////////////////////////////////////////////////////////////////////////
+nsresult MenuBarListener::KeyDown(Event* aKeyEvent) {
+  // handlers shouldn't be triggered by non-trusted events.
+  if (!aKeyEvent || !aKeyEvent->IsTrusted()) {
+    return NS_OK;
+  }
+
+  RefPtr<KeyboardEvent> keyEvent = aKeyEvent->AsKeyboardEvent();
+  if (!keyEvent) {
+    return NS_OK;
+  }
+
+  uint32_t theChar = keyEvent->KeyCode();
+  uint16_t eventPhase = keyEvent->EventPhase();
+  bool capturing = (eventPhase == dom::Event_Binding::CAPTURING_PHASE);
+
+#ifndef XP_MACOSX
+  if (capturing && !mAccessKeyDown && theChar == NS_VK_F10 &&
+      (keyEvent->GetModifiersForMenuAccessKey() & ~MODIFIER_CONTROL) == 0) {
+    ReserveKeyIfNeeded(aKeyEvent);
+  }
+#endif
+
+  const auto accessKey = LookAndFeel::GetMenuAccessKey();
+  if (accessKey && StaticPrefs::ui_key_menuAccessKeyFocuses()) {
+    bool defaultPrevented = aKeyEvent->DefaultPrevented();
+
+    // No other modifiers can be down.
+    // Especially CTRL.  CTRL+ALT == AltGR, and we'll break on non-US
+    // enhanced 102-key keyboards if we don't check this.
+    bool isAccessKeyDownEvent =
+        (theChar == accessKey &&
+         (keyEvent->GetModifiersForMenuAccessKey() &
+          ~LookAndFeel::GetMenuAccessKeyModifiers()) == 0);
+
+    if (!capturing && !mAccessKeyDown) {
+      // If accesskey isn't being pressed and the key isn't the accesskey,
+      // ignore the event.
+      if (!isAccessKeyDownEvent) {
+        return NS_OK;
+      }
+
+      // Otherwise, accept the accesskey state.
+      mAccessKeyDown = true;
+      // If default is prevented already, cancel the access key down.
+      mAccessKeyDownCanceled = defaultPrevented;
+      return NS_OK;
+    }
+
+    // If the pressed accesskey was canceled already or the event was
+    // consumed already, ignore the event.
+    if (mAccessKeyDownCanceled || defaultPrevented) {
+      return NS_OK;
+    }
+
+    // Some key other than the access key just went down,
+    // so we won't activate the menu bar when the access key is released.
+    mAccessKeyDownCanceled = !isAccessKeyDownEvent;
+  }
+
+  if (capturing && accessKey) {
+    if (GetMenuForKeyEvent(*keyEvent)) {
+      ReserveKeyIfNeeded(aKeyEvent);
+    }
+  }
+
+  return NS_OK;  // means I am NOT consuming event
+}
+
+////////////////////////////////////////////////////////////////////////
+
+nsresult MenuBarListener::Blur(Event* aEvent) {
+  if (!IsMenuOpen() && mMenuBar->IsActive()) {
+    ToggleMenuActiveState(ByKeyboard::No);
+    mAccessKeyDown = false;
+    mAccessKeyDownCanceled = false;
+  }
+  return NS_OK;  // means I am NOT consuming event
+}
+
+////////////////////////////////////////////////////////////////////////
+
+nsresult MenuBarListener::OnWindowDeactivated(Event* aEvent) {
+  // Reset the accesskey state because we cannot receive the keyup event for
+  // the pressing accesskey.
+  mAccessKeyDown = false;
+  mAccessKeyDownCanceled = false;
+  return NS_OK;  // means I am NOT consuming event
+}
+
+bool MenuBarListener::IsMenuOpen() const {
+  auto* activeChild = mMenuBar->GetActiveMenuChild();
+  return activeChild && activeChild->IsMenuPopupOpen();
+}
+
+////////////////////////////////////////////////////////////////////////
+nsresult MenuBarListener::MouseDown(Event* aMouseEvent) {
+  // NOTE: MouseDown method listens all phases
+
+  // Even if the mousedown event is canceled, it means the user don't want
+  // to activate the menu.  Therefore, we need to record it at capturing (or
+  // target) phase.
+  if (mAccessKeyDown) {
+    mAccessKeyDownCanceled = true;
+  }
+
+  // Don't do anything at capturing phase, any behavior should be cancelable.
+  if (aMouseEvent->EventPhase() == dom::Event_Binding::CAPTURING_PHASE) {
+    return NS_OK;
+  }
+
+  if (!IsMenuOpen() && mMenuBar->IsActive()) {
+    ToggleMenuActiveState(ByKeyboard::No);
+  }
+
+  return NS_OK;  // means I am NOT consuming event
+}
+
+////////////////////////////////////////////////////////////////////////
+
+nsresult MenuBarListener::Fullscreen(Event* aEvent) {
+  if (mMenuBar->IsActive()) {
+    ToggleMenuActiveState(ByKeyboard::No);
+  }
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////
+MOZ_CAN_RUN_SCRIPT_BOUNDARY nsresult
+MenuBarListener::HandleEvent(Event* aEvent) {
+  // If the menu bar is collapsed, don't do anything.
+  if (!mMenuBar || !mMenuBar->GetPrimaryFrame() ||
+      !mMenuBar->GetPrimaryFrame()->StyleVisibility()->IsVisible()) {
+    return NS_OK;
+  }
+
+  nsAutoString eventType;
+  aEvent->GetType(eventType);
+
+  if (eventType.EqualsLiteral("keyup")) {
+    return KeyUp(aEvent);
+  }
+  if (eventType.EqualsLiteral("keydown")) {
+    return KeyDown(aEvent);
+  }
+  if (eventType.EqualsLiteral("keypress")) {
+    return KeyPress(aEvent);
+  }
+  if (eventType.EqualsLiteral("mozaccesskeynotfound")) {
+    return KeyPress(aEvent);
+  }
+  if (eventType.EqualsLiteral("blur")) {
+    return Blur(aEvent);
+  }
+  if (eventType.EqualsLiteral("deactivate")) {
+    return OnWindowDeactivated(aEvent);
+  }
+  if (eventType.EqualsLiteral("mousedown")) {
+    return MouseDown(aEvent);
+  }
+  if (eventType.EqualsLiteral("MozDOMFullscreen:Entered")) {
+    return Fullscreen(aEvent);
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unexpected eventType");
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/MenuBarListener.h b/dom/xul/MenuBarListener.h
new file mode 100644
index 0000000000..2985a77079
--- /dev/null
+++ b/dom/xul/MenuBarListener.h
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+#ifndef mozilla_dom_MenuBarListener_h
+#define mozilla_dom_MenuBarListener_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/EventForwards.h"
+#include "nsIDOMEventListener.h"
+
+namespace mozilla::dom {
+class Document;
+class EventTarget;
+class KeyboardEvent;
+class XULMenuBarElement;
+class XULButtonElement;
+
+class MenuBarListener final : public nsIDOMEventListener {
+ public:
+  explicit MenuBarListener(XULMenuBarElement&);
+
+  NS_DECL_ISUPPORTS
+
+  NS_DECL_NSIDOMEVENTLISTENER
+
+  // Should be called when unbound from the document and so on.
+  void Detach();
+
+ protected:
+  virtual ~MenuBarListener();
+
+  bool IsMenuOpen() const;
+
+  MOZ_CAN_RUN_SCRIPT nsresult KeyUp(Event* aMouseEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult KeyDown(Event* aMouseEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult KeyPress(Event* aMouseEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult Blur(Event* aEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult OnWindowDeactivated(Event* aEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult MouseDown(Event* aMouseEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult Fullscreen(Event* aEvent);
+
+  /**
+   * Given a key event for an Alt+shortcut combination,
+   * return the menu, if any, that would be opened. If aPeek
+   * is false, then play a beep and deactivate the menubar on Windows.
+   */
+  XULButtonElement* GetMenuForKeyEvent(KeyboardEvent& aKeyEvent);
+
+  /**
+   * Call MarkAsReservedByChrome if the user's preferences indicate that
+   * the key should be chrome-only.
+   */
+  void ReserveKeyIfNeeded(Event* aKeyEvent);
+
+  // This should only be called by the MenuBarListener during event dispatch.
+  enum class ByKeyboard : bool { No, Yes };
+  MOZ_CAN_RUN_SCRIPT void ToggleMenuActiveState(ByKeyboard);
+
+  bool Destroyed() const { return !mMenuBar; }
+
+  // The menu bar object. Safe because it keeps us alive.
+  XULMenuBarElement* mMenuBar;
+  // The event target to listen to the events.
+  //
+  // Weak reference is safe because we clear the listener on unbind from the
+  // document.
+  Document* mEventTarget;
+  // Whether or not the ALT key is currently down.
+  bool mAccessKeyDown = false;
+  // Whether or not the ALT key down is canceled by other action.
+  bool mAccessKeyDownCanceled = false;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // #ifndef mozilla_dom_MenuBarListener_h
diff --git a/dom/xul/XULBroadcastManager.cpp b/dom/xul/XULBroadcastManager.cpp
new file mode 100644
index 0000000000..c9afefd0c8
--- /dev/null
+++ b/dom/xul/XULBroadcastManager.cpp
@@ -0,0 +1,589 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=4 sw=2 et tw=80: */
+/* 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 "XULBroadcastManager.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/Logging.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "nsXULElement.h"
+
+struct BroadcastListener {
+  nsWeakPtr mListener;
+  RefPtr<nsAtom> mAttribute;
+};
+
+struct BroadcasterMapEntry : public PLDHashEntryHdr {
+  mozilla::dom::Element* mBroadcaster;  // [WEAK]
+  nsTArray<BroadcastListener*>
+      mListeners;  // [OWNING] of BroadcastListener objects
+};
+
+struct nsAttrNameInfo {
+  nsAttrNameInfo(int32_t aNamespaceID, nsAtom* aName, nsAtom* aPrefix)
+      : mNamespaceID(aNamespaceID), mName(aName), mPrefix(aPrefix) {}
+  nsAttrNameInfo(const nsAttrNameInfo& aOther) = delete;
+  nsAttrNameInfo(nsAttrNameInfo&& aOther) = default;
+
+  int32_t mNamespaceID;
+  RefPtr<nsAtom> mName;
+  RefPtr<nsAtom> mPrefix;
+};
+
+static void ClearBroadcasterMapEntry(PLDHashTable* aTable,
+                                     PLDHashEntryHdr* aEntry) {
+  BroadcasterMapEntry* entry = static_cast<BroadcasterMapEntry*>(aEntry);
+  for (size_t i = entry->mListeners.Length() - 1; i != (size_t)-1; --i) {
+    delete entry->mListeners[i];
+  }
+  entry->mListeners.Clear();
+
+  // N.B. that we need to manually run the dtor because we
+  // constructed the nsTArray object in-place.
+  entry->mListeners.~nsTArray<BroadcastListener*>();
+}
+
+static bool CanBroadcast(int32_t aNameSpaceID, nsAtom* aAttribute) {
+  // Don't push changes to the |id|, |persist|, |command| or
+  // |observes| attribute.
+  if (aNameSpaceID == kNameSpaceID_None) {
+    if ((aAttribute == nsGkAtoms::id) || (aAttribute == nsGkAtoms::persist) ||
+        (aAttribute == nsGkAtoms::command) ||
+        (aAttribute == nsGkAtoms::observes)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+namespace mozilla::dom {
+static LazyLogModule sXULBroadCastManager("XULBroadcastManager");
+
+class XULBroadcastManager::nsDelayedBroadcastUpdate {
+ public:
+  nsDelayedBroadcastUpdate(Element* aBroadcaster, Element* aListener,
+                           const nsAString& aAttr)
+      : mBroadcaster(aBroadcaster),
+        mListener(aListener),
+        mAttr(aAttr),
+        mSetAttr(false),
+        mNeedsAttrChange(false) {}
+
+  nsDelayedBroadcastUpdate(Element* aBroadcaster, Element* aListener,
+                           nsAtom* aAttrName, const nsAString& aAttr,
+                           bool aSetAttr, bool aNeedsAttrChange)
+      : mBroadcaster(aBroadcaster),
+        mListener(aListener),
+        mAttr(aAttr),
+        mAttrName(aAttrName),
+        mSetAttr(aSetAttr),
+        mNeedsAttrChange(aNeedsAttrChange) {}
+
+  nsDelayedBroadcastUpdate(const nsDelayedBroadcastUpdate& aOther) = delete;
+  nsDelayedBroadcastUpdate(nsDelayedBroadcastUpdate&& aOther) = default;
+
+  RefPtr<Element> mBroadcaster;
+  RefPtr<Element> mListener;
+  // Note if mAttrName isn't used, this is the name of the attr, otherwise
+  // this is the value of the attribute.
+  nsString mAttr;
+  RefPtr<nsAtom> mAttrName;
+  bool mSetAttr;
+  bool mNeedsAttrChange;
+
+  class Comparator {
+   public:
+    static bool Equals(const nsDelayedBroadcastUpdate& a,
+                       const nsDelayedBroadcastUpdate& b) {
+      return a.mBroadcaster == b.mBroadcaster && a.mListener == b.mListener &&
+             a.mAttrName == b.mAttrName;
+    }
+  };
+};
+
+/* static */
+bool XULBroadcastManager::MayNeedListener(const Element& aElement) {
+  if (aElement.NodeInfo()->Equals(nsGkAtoms::observes, kNameSpaceID_XUL)) {
+    return true;
+  }
+  if (aElement.HasAttr(nsGkAtoms::observes)) {
+    return true;
+  }
+  if (aElement.HasAttr(nsGkAtoms::command) &&
+      !(aElement.NodeInfo()->Equals(nsGkAtoms::menuitem, kNameSpaceID_XUL) ||
+        aElement.NodeInfo()->Equals(nsGkAtoms::key, kNameSpaceID_XUL))) {
+    return true;
+  }
+  return false;
+}
+
+XULBroadcastManager::XULBroadcastManager(Document* aDocument)
+    : mDocument(aDocument),
+      mBroadcasterMap(nullptr),
+      mHandlingDelayedAttrChange(false),
+      mHandlingDelayedBroadcasters(false) {}
+
+XULBroadcastManager::~XULBroadcastManager() { delete mBroadcasterMap; }
+
+void XULBroadcastManager::DropDocumentReference(void) { mDocument = nullptr; }
+
+void XULBroadcastManager::SynchronizeBroadcastListener(Element* aBroadcaster,
+                                                       Element* aListener,
+                                                       const nsAString& aAttr) {
+  if (!nsContentUtils::IsSafeToRunScript()) {
+    mDelayedBroadcasters.EmplaceBack(aBroadcaster, aListener, aAttr);
+    MaybeBroadcast();
+    return;
+  }
+  bool notify = mHandlingDelayedBroadcasters;
+
+  if (aAttr.EqualsLiteral("*")) {
+    uint32_t count = aBroadcaster->GetAttrCount();
+    nsTArray<nsAttrNameInfo> attributes(count);
+    for (uint32_t i = 0; i < count; ++i) {
+      const nsAttrName* attrName = aBroadcaster->GetAttrNameAt(i);
+      int32_t nameSpaceID = attrName->NamespaceID();
+      nsAtom* name = attrName->LocalName();
+
+      // _Don't_ push the |id|, |ref|, or |persist| attribute's value!
+      if (!CanBroadcast(nameSpaceID, name)) continue;
+
+      attributes.AppendElement(
+          nsAttrNameInfo(nameSpaceID, name, attrName->GetPrefix()));
+    }
+
+    count = attributes.Length();
+    while (count-- > 0) {
+      int32_t nameSpaceID = attributes[count].mNamespaceID;
+      nsAtom* name = attributes[count].mName;
+      nsAutoString value;
+      if (aBroadcaster->GetAttr(nameSpaceID, name, value)) {
+        aListener->SetAttr(nameSpaceID, name, attributes[count].mPrefix, value,
+                           notify);
+      }
+
+#if 0
+            // XXX we don't fire the |onbroadcast| handler during
+            // initial hookup: doing so would potentially run the
+            // |onbroadcast| handler before the |onload| handler,
+            // which could define JS properties that mask XBL
+            // properties, etc.
+            ExecuteOnBroadcastHandlerFor(aBroadcaster, aListener, name);
+#endif
+    }
+  } else {
+    // Find out if the attribute is even present at all.
+    RefPtr<nsAtom> name = NS_Atomize(aAttr);
+
+    nsAutoString value;
+    if (aBroadcaster->GetAttr(name, value)) {
+      aListener->SetAttr(kNameSpaceID_None, name, value, notify);
+    } else {
+      aListener->UnsetAttr(kNameSpaceID_None, name, notify);
+    }
+
+#if 0
+        // XXX we don't fire the |onbroadcast| handler during initial
+        // hookup: doing so would potentially run the |onbroadcast|
+        // handler before the |onload| handler, which could define JS
+        // properties that mask XBL properties, etc.
+        ExecuteOnBroadcastHandlerFor(aBroadcaster, aListener, name);
+#endif
+  }
+}
+
+void XULBroadcastManager::AddListenerFor(Element& aBroadcaster,
+                                         Element& aListener,
+                                         const nsAString& aAttr,
+                                         ErrorResult& aRv) {
+  if (!mDocument) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
+  }
+
+  nsresult rv = nsContentUtils::CheckSameOrigin(mDocument, &aBroadcaster);
+
+  if (NS_FAILED(rv)) {
+    aRv.Throw(rv);
+    return;
+  }
+
+  rv = nsContentUtils::CheckSameOrigin(mDocument, &aListener);
+
+  if (NS_FAILED(rv)) {
+    aRv.Throw(rv);
+    return;
+  }
+
+  static const PLDHashTableOps gOps = {
+      PLDHashTable::HashVoidPtrKeyStub, PLDHashTable::MatchEntryStub,
+      PLDHashTable::MoveEntryStub, ClearBroadcasterMapEntry, nullptr};
+
+  if (!mBroadcasterMap) {
+    mBroadcasterMap = new PLDHashTable(&gOps, sizeof(BroadcasterMapEntry));
+  }
+
+  auto entry =
+      static_cast<BroadcasterMapEntry*>(mBroadcasterMap->Search(&aBroadcaster));
+  if (!entry) {
+    entry = static_cast<BroadcasterMapEntry*>(
+        mBroadcasterMap->Add(&aBroadcaster, fallible));
+
+    if (!entry) {
+      aRv.Throw(NS_ERROR_OUT_OF_MEMORY);
+      return;
+    }
+
+    entry->mBroadcaster = &aBroadcaster;
+
+    // N.B. placement new to construct the nsTArray object in-place
+    new (&entry->mListeners) nsTArray<BroadcastListener*>();
+  }
+
+  // Only add the listener if it's not there already!
+  RefPtr<nsAtom> attr = NS_Atomize(aAttr);
+
+  for (size_t i = entry->mListeners.Length() - 1; i != (size_t)-1; --i) {
+    BroadcastListener* bl = entry->mListeners[i];
+    nsCOMPtr<Element> blListener = do_QueryReferent(bl->mListener);
+
+    if (blListener == &aListener && bl->mAttribute == attr) return;
+  }
+
+  BroadcastListener* bl = new BroadcastListener;
+  bl->mListener = do_GetWeakReference(&aListener);
+  bl->mAttribute = attr;
+
+  entry->mListeners.AppendElement(bl);
+
+  SynchronizeBroadcastListener(&aBroadcaster, &aListener, aAttr);
+}
+
+void XULBroadcastManager::RemoveListenerFor(Element& aBroadcaster,
+                                            Element& aListener,
+                                            const nsAString& aAttr) {
+  // If we haven't added any broadcast listeners, then there sure
+  // aren't any to remove.
+  if (!mBroadcasterMap) return;
+
+  auto entry =
+      static_cast<BroadcasterMapEntry*>(mBroadcasterMap->Search(&aBroadcaster));
+  if (entry) {
+    RefPtr<nsAtom> attr = NS_Atomize(aAttr);
+    for (size_t i = entry->mListeners.Length() - 1; i != (size_t)-1; --i) {
+      BroadcastListener* bl = entry->mListeners[i];
+      nsCOMPtr<Element> blListener = do_QueryReferent(bl->mListener);
+
+      if (blListener == &aListener && bl->mAttribute == attr) {
+        entry->mListeners.RemoveElementAt(i);
+        delete bl;
+
+        if (entry->mListeners.IsEmpty()) mBroadcasterMap->RemoveEntry(entry);
+
+        break;
+      }
+    }
+  }
+}
+
+nsresult XULBroadcastManager::ExecuteOnBroadcastHandlerFor(
+    Element* aBroadcaster, Element* aListener, nsAtom* aAttr) {
+  if (!mDocument) {
+    return NS_OK;
+  }
+  // Now we execute the onchange handler in the context of the
+  // observer. We need to find the observer in order to
+  // execute the handler.
+
+  for (nsCOMPtr<nsIContent> child = aListener->GetFirstChild(); child;
+       child = child->GetNextSibling()) {
+    // Look for an <observes> element beneath the listener. This
+    // ought to have an |element| attribute that refers to
+    // aBroadcaster, and an |attribute| element that tells us what
+    // attriubtes we're listening for.
+    if (!child->IsXULElement(nsGkAtoms::observes)) continue;
+
+    // Is this the element that was listening to us?
+    nsAutoString listeningToID;
+    child->AsElement()->GetAttr(nsGkAtoms::element, listeningToID);
+
+    nsAutoString broadcasterID;
+    aBroadcaster->GetAttr(nsGkAtoms::id, broadcasterID);
+
+    if (listeningToID != broadcasterID) continue;
+
+    // We are observing the broadcaster, but is this the right
+    // attribute?
+    nsAutoString listeningToAttribute;
+    child->AsElement()->GetAttr(nsGkAtoms::attribute, listeningToAttribute);
+
+    if (!aAttr->Equals(listeningToAttribute) &&
+        !listeningToAttribute.EqualsLiteral("*")) {
+      continue;
+    }
+
+    // This is the right <observes> element. Execute the
+    // |onbroadcast| event handler
+    WidgetEvent event(true, eXULBroadcast);
+
+    if (RefPtr<nsPresContext> presContext = mDocument->GetPresContext()) {
+      // Handle the DOM event
+      nsEventStatus status = nsEventStatus_eIgnore;
+      EventDispatcher::Dispatch(child, presContext, &event, nullptr, &status);
+    }
+  }
+
+  return NS_OK;
+}
+
+void XULBroadcastManager::AttributeChanged(Element* aElement,
+                                           int32_t aNameSpaceID,
+                                           nsAtom* aAttribute) {
+  if (!mDocument) {
+    return;
+  }
+  NS_ASSERTION(aElement->OwnerDoc() == mDocument, "unexpected doc");
+
+  // Synchronize broadcast listeners
+  if (mBroadcasterMap && CanBroadcast(aNameSpaceID, aAttribute)) {
+    auto entry =
+        static_cast<BroadcasterMapEntry*>(mBroadcasterMap->Search(aElement));
+
+    if (entry) {
+      // We've got listeners: push the value.
+      nsAutoString value;
+      bool attrSet = aElement->GetAttr(aAttribute, value);
+
+      for (size_t i = entry->mListeners.Length() - 1; i != (size_t)-1; --i) {
+        BroadcastListener* bl = entry->mListeners[i];
+        if ((bl->mAttribute == aAttribute) ||
+            (bl->mAttribute == nsGkAtoms::_asterisk)) {
+          nsCOMPtr<Element> listenerEl = do_QueryReferent(bl->mListener);
+          if (listenerEl) {
+            nsAutoString currentValue;
+            bool hasAttr = listenerEl->GetAttr(aAttribute, currentValue);
+            // We need to update listener only if we're
+            // (1) removing an existing attribute,
+            // (2) adding a new attribute or
+            // (3) changing the value of an attribute.
+            bool needsAttrChange =
+                attrSet != hasAttr || !value.Equals(currentValue);
+            nsDelayedBroadcastUpdate delayedUpdate(aElement, listenerEl,
+                                                   aAttribute, value, attrSet,
+                                                   needsAttrChange);
+
+            size_t index = mDelayedAttrChangeBroadcasts.IndexOf(
+                delayedUpdate, 0, nsDelayedBroadcastUpdate::Comparator());
+            if (index != mDelayedAttrChangeBroadcasts.NoIndex) {
+              if (mHandlingDelayedAttrChange) {
+                NS_WARNING("Broadcasting loop!");
+                continue;
+              }
+              mDelayedAttrChangeBroadcasts.RemoveElementAt(index);
+            }
+
+            mDelayedAttrChangeBroadcasts.AppendElement(
+                std::move(delayedUpdate));
+          }
+        }
+      }
+    }
+  }
+}
+
+void XULBroadcastManager::MaybeBroadcast() {
+  // Only broadcast when not in an update and when safe to run scripts.
+  if (mDocument && mDocument->UpdateNestingLevel() == 0 &&
+      (mDelayedAttrChangeBroadcasts.Length() ||
+       mDelayedBroadcasters.Length())) {
+    if (!nsContentUtils::IsSafeToRunScript()) {
+      if (mDocument) {
+        nsContentUtils::AddScriptRunner(
+            NewRunnableMethod("dom::XULBroadcastManager::MaybeBroadcast", this,
+                              &XULBroadcastManager::MaybeBroadcast));
+      }
+      return;
+    }
+    if (!mHandlingDelayedAttrChange) {
+      mHandlingDelayedAttrChange = true;
+      for (uint32_t i = 0; i < mDelayedAttrChangeBroadcasts.Length(); ++i) {
+        RefPtr<nsAtom> attrName = mDelayedAttrChangeBroadcasts[i].mAttrName;
+        RefPtr<Element> listener = mDelayedAttrChangeBroadcasts[i].mListener;
+        if (mDelayedAttrChangeBroadcasts[i].mNeedsAttrChange) {
+          const nsString& value = mDelayedAttrChangeBroadcasts[i].mAttr;
+          if (mDelayedAttrChangeBroadcasts[i].mSetAttr) {
+            listener->SetAttr(kNameSpaceID_None, attrName, value, true);
+          } else {
+            listener->UnsetAttr(kNameSpaceID_None, attrName, true);
+          }
+        }
+        RefPtr<Element> broadcaster =
+            mDelayedAttrChangeBroadcasts[i].mBroadcaster;
+        ExecuteOnBroadcastHandlerFor(broadcaster, listener, attrName);
+      }
+      mDelayedAttrChangeBroadcasts.Clear();
+      mHandlingDelayedAttrChange = false;
+    }
+
+    uint32_t length = mDelayedBroadcasters.Length();
+    if (length) {
+      bool oldValue = mHandlingDelayedBroadcasters;
+      mHandlingDelayedBroadcasters = true;
+      nsTArray<nsDelayedBroadcastUpdate> delayedBroadcasters =
+          std::move(mDelayedBroadcasters);
+      for (uint32_t i = 0; i < length; ++i) {
+        SynchronizeBroadcastListener(delayedBroadcasters[i].mBroadcaster,
+                                     delayedBroadcasters[i].mListener,
+                                     delayedBroadcasters[i].mAttr);
+      }
+      mHandlingDelayedBroadcasters = oldValue;
+    }
+  }
+}
+
+nsresult XULBroadcastManager::FindBroadcaster(Element* aElement,
+                                              Element** aListener,
+                                              nsString& aBroadcasterID,
+                                              nsString& aAttribute,
+                                              Element** aBroadcaster) {
+  NodeInfo* ni = aElement->NodeInfo();
+  *aListener = nullptr;
+  *aBroadcaster = nullptr;
+
+  if (ni->Equals(nsGkAtoms::observes, kNameSpaceID_XUL)) {
+    // It's an <observes> element, which means that the actual
+    // listener is the _parent_ node. This element should have an
+    // 'element' attribute that specifies the ID of the
+    // broadcaster element, and an 'attribute' element, which
+    // specifies the name of the attribute to observe.
+    nsIContent* parent = aElement->GetParent();
+    if (!parent) {
+      // <observes> is the root element
+      return NS_FINDBROADCASTER_NOT_FOUND;
+    }
+
+    *aListener = Element::FromNode(parent);
+    NS_IF_ADDREF(*aListener);
+
+    aElement->GetAttr(nsGkAtoms::element, aBroadcasterID);
+    if (aBroadcasterID.IsEmpty()) {
+      return NS_FINDBROADCASTER_NOT_FOUND;
+    }
+    aElement->GetAttr(nsGkAtoms::attribute, aAttribute);
+  } else {
+    // It's a generic element, which means that we'll use the
+    // value of the 'observes' attribute to determine the ID of
+    // the broadcaster element, and we'll watch _all_ of its
+    // values.
+    aElement->GetAttr(nsGkAtoms::observes, aBroadcasterID);
+
+    // Bail if there's no aBroadcasterID
+    if (aBroadcasterID.IsEmpty()) {
+      // Try the command attribute next.
+      aElement->GetAttr(nsGkAtoms::command, aBroadcasterID);
+      if (!aBroadcasterID.IsEmpty()) {
+        // We've got something in the command attribute.  We
+        // only treat this as a normal broadcaster if we are
+        // not a menuitem or a key.
+
+        if (ni->Equals(nsGkAtoms::menuitem, kNameSpaceID_XUL) ||
+            ni->Equals(nsGkAtoms::key, kNameSpaceID_XUL)) {
+          return NS_FINDBROADCASTER_NOT_FOUND;
+        }
+      } else {
+        return NS_FINDBROADCASTER_NOT_FOUND;
+      }
+    }
+
+    *aListener = aElement;
+    NS_ADDREF(*aListener);
+
+    aAttribute.Assign('*');
+  }
+
+  // Make sure we got a valid listener.
+  NS_ENSURE_TRUE(*aListener, NS_ERROR_UNEXPECTED);
+
+  // Try to find the broadcaster element in the document.
+  Document* doc = aElement->GetComposedDoc();
+  if (doc) {
+    *aBroadcaster = doc->GetElementById(aBroadcasterID);
+  }
+
+  // The broadcaster element is missing.
+  if (!*aBroadcaster) {
+    return NS_FINDBROADCASTER_NOT_FOUND;
+  }
+
+  NS_ADDREF(*aBroadcaster);
+
+  return NS_FINDBROADCASTER_FOUND;
+}
+
+nsresult XULBroadcastManager::UpdateListenerHookup(Element* aElement,
+                                                   HookupAction aAction) {
+  // Resolve a broadcaster hookup. Look at the element that we're
+  // trying to resolve: it could be an '<observes>' element, or just
+  // a vanilla element with an 'observes' attribute on it.
+  nsresult rv;
+
+  nsCOMPtr<Element> listener;
+  nsAutoString broadcasterID;
+  nsAutoString attribute;
+  nsCOMPtr<Element> broadcaster;
+
+  rv = FindBroadcaster(aElement, getter_AddRefs(listener), broadcasterID,
+                       attribute, getter_AddRefs(broadcaster));
+  switch (rv) {
+    case NS_FINDBROADCASTER_NOT_FOUND:
+      return NS_OK;
+    case NS_FINDBROADCASTER_FOUND:
+      break;
+    default:
+      return rv;
+  }
+
+  NS_ENSURE_ARG(broadcaster && listener);
+  if (aAction == eHookupAdd) {
+    ErrorResult domRv;
+    AddListenerFor(*broadcaster, *listener, attribute, domRv);
+    if (domRv.Failed()) {
+      return domRv.StealNSResult();
+    }
+  } else {
+    RemoveListenerFor(*broadcaster, *listener, attribute);
+  }
+
+  // Tell the world we succeeded
+  if (MOZ_LOG_TEST(sXULBroadCastManager, LogLevel::Debug)) {
+    nsCOMPtr<nsIContent> content = listener;
+    NS_ASSERTION(content != nullptr, "not an nsIContent");
+    if (!content) {
+      return rv;
+    }
+
+    nsAutoCString attributeC, broadcasteridC;
+    LossyCopyUTF16toASCII(attribute, attributeC);
+    LossyCopyUTF16toASCII(broadcasterID, broadcasteridC);
+    MOZ_LOG(sXULBroadCastManager, LogLevel::Debug,
+            ("xul: broadcaster hookup <%s attribute='%s'> to %s",
+             nsAtomCString(content->NodeInfo()->NameAtom()).get(),
+             attributeC.get(), broadcasteridC.get()));
+  }
+
+  return NS_OK;
+}
+
+nsresult XULBroadcastManager::AddListener(Element* aElement) {
+  return UpdateListenerHookup(aElement, eHookupAdd);
+}
+
+nsresult XULBroadcastManager::RemoveListener(Element* aElement) {
+  return UpdateListenerHookup(aElement, eHookupRemove);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULBroadcastManager.h b/dom/xul/XULBroadcastManager.h
new file mode 100644
index 0000000000..77fe6168eb
--- /dev/null
+++ b/dom/xul/XULBroadcastManager.h
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_dom_XULBroadcastManager_h
+#define mozilla_dom_XULBroadcastManager_h
+
+#include "nsAtom.h"
+#include "nsTArray.h"
+
+class PLDHashTable;
+class nsXULElement;
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+class Document;
+class Element;
+
+class XULBroadcastManager final {
+ public:
+  explicit XULBroadcastManager(Document* aDocument);
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(XULBroadcastManager)
+
+  /**
+   * Checks whether an element uses any of the special broadcaster attributes
+   * or is an observes element. This mimics the logic in FindBroadcaster, but
+   * is intended to be a lighter weight check and doesn't actually guarantee
+   * that the element will need a listener.
+   */
+  static bool MayNeedListener(const Element& aElement);
+
+  nsresult AddListener(Element* aElement);
+  nsresult RemoveListener(Element* aElement);
+  void AttributeChanged(Element* aElement, int32_t aNameSpaceID,
+                        nsAtom* aAttribute);
+  // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void MaybeBroadcast();
+  void DropDocumentReference();  // notification that doc is going away
+ protected:
+  enum HookupAction { eHookupAdd = 0, eHookupRemove };
+
+  nsresult UpdateListenerHookup(Element* aElement, HookupAction aAction);
+
+  void RemoveListenerFor(Element& aBroadcaster, Element& aListener,
+                         const nsAString& aAttr);
+  void AddListenerFor(Element& aBroadcaster, Element& aListener,
+                      const nsAString& aAttr, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT nsresult ExecuteOnBroadcastHandlerFor(
+      Element* aBroadcaster, Element* aListener, nsAtom* aAttr);
+  // The out params of FindBroadcaster only have values that make sense when
+  // the method returns NS_FINDBROADCASTER_FOUND.  In all other cases, the
+  // values of the out params should not be relied on (though *aListener and
+  // *aBroadcaster do need to be released if non-null, of course).
+  nsresult FindBroadcaster(Element* aElement, Element** aListener,
+                           nsString& aBroadcasterID, nsString& aAttribute,
+                           Element** aBroadcaster);
+
+  void SynchronizeBroadcastListener(Element* aBroadcaster, Element* aListener,
+                                    const nsAString& aAttr);
+
+  // This reference is nulled by the Document in it's destructor through
+  // DropDocumentReference().
+  Document* MOZ_NON_OWNING_REF mDocument;
+
+  /**
+   * A map from a broadcaster element to a list of listener elements.
+   */
+  PLDHashTable* mBroadcasterMap;
+
+  class nsDelayedBroadcastUpdate;
+  nsTArray<nsDelayedBroadcastUpdate> mDelayedBroadcasters;
+  nsTArray<nsDelayedBroadcastUpdate> mDelayedAttrChangeBroadcasts;
+  bool mHandlingDelayedAttrChange;
+  bool mHandlingDelayedBroadcasters;
+
+ private:
+  ~XULBroadcastManager();
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_XULBroadcastManager_h
diff --git a/dom/xul/XULButtonElement.cpp b/dom/xul/XULButtonElement.cpp
new file mode 100644
index 0000000000..fa9ee28628
--- /dev/null
+++ b/dom/xul/XULButtonElement.cpp
@@ -0,0 +1,798 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "XULButtonElement.h"
+#include "XULMenuParentElement.h"
+#include "XULPopupElement.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/EventStateManager.h"
+#include "mozilla/LookAndFeel.h"
+#include "mozilla/TextEvents.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/dom/MouseEventBinding.h"
+#include "mozilla/dom/NameSpaceConstants.h"
+#include "mozilla/dom/AncestorIterator.h"
+#include "mozilla/dom/XULMenuBarElement.h"
+#include "nsGkAtoms.h"
+#include "nsITimer.h"
+#include "nsLayoutUtils.h"
+#include "nsCaseTreatment.h"
+#include "nsChangeHint.h"
+#include "nsMenuPopupFrame.h"
+#include "nsPlaceholderFrame.h"
+#include "nsPresContext.h"
+#include "nsXULPopupManager.h"
+#include "nsIDOMXULButtonElement.h"
+#include "nsISound.h"
+
+namespace mozilla::dom {
+
+XULButtonElement::XULButtonElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+    : nsXULElement(std::move(aNodeInfo)),
+      mIsAlwaysMenu(IsAnyOfXULElements(nsGkAtoms::menu, nsGkAtoms::menulist,
+                                       nsGkAtoms::menuitem)) {}
+
+XULButtonElement::~XULButtonElement() {
+  StopBlinking();
+  KillMenuOpenTimer();
+}
+
+nsChangeHint XULButtonElement::GetAttributeChangeHint(const nsAtom* aAttribute,
+                                                      int32_t aModType) const {
+  if (aAttribute == nsGkAtoms::type &&
+      IsAnyOfXULElements(nsGkAtoms::button, nsGkAtoms::toolbarbutton)) {
+    // type=menu switches to a menu frame.
+    return nsChangeHint_ReconstructFrame;
+  }
+  return nsXULElement::GetAttributeChangeHint(aAttribute, aModType);
+}
+
+// This global flag is used to record the timestamp when a menu was opened or
+// closed and is used to ignore the mousemove and mouseup events that would fire
+// on the menu after the mousedown occurred.
+static TimeStamp gMenuJustOpenedOrClosedTime = TimeStamp();
+
+void XULButtonElement::PopupOpened() {
+  if (!IsMenu()) {
+    return;
+  }
+  gMenuJustOpenedOrClosedTime = TimeStamp::Now();
+  SetAttr(kNameSpaceID_None, nsGkAtoms::open, u"true"_ns, true);
+}
+
+void XULButtonElement::PopupClosed(bool aDeselectMenu) {
+  if (!IsMenu()) {
+    return;
+  }
+  nsContentUtils::AddScriptRunner(
+      new nsUnsetAttrRunnable(this, nsGkAtoms::open));
+
+  if (aDeselectMenu) {
+    if (RefPtr<XULMenuParentElement> parent = GetMenuParent()) {
+      if (parent->GetActiveMenuChild() == this) {
+        parent->SetActiveMenuChild(nullptr);
+      }
+    }
+  }
+}
+
+bool XULButtonElement::IsMenuActive() const {
+  if (XULMenuParentElement* menu = GetMenuParent()) {
+    return menu->GetActiveMenuChild() == this;
+  }
+  return false;
+}
+
+void XULButtonElement::HandleEnterKeyPress(WidgetEvent& aEvent) {
+  if (IsDisabled()) {
+#ifdef XP_WIN
+    if (XULPopupElement* popup = GetContainingPopupElement()) {
+      if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+        pm->HidePopup(
+            popup, {HidePopupOption::HideChain, HidePopupOption::DeselectMenu,
+                    HidePopupOption::Async});
+      }
+    }
+#endif
+    return;
+  }
+  if (IsMenuPopupOpen()) {
+    return;
+  }
+  // The enter key press applies to us.
+  if (IsMenuItem()) {
+    ExecuteMenu(aEvent);
+  } else {
+    OpenMenuPopup(true);
+  }
+}
+
+bool XULButtonElement::IsMenuPopupOpen() {
+  nsMenuPopupFrame* popupFrame = GetMenuPopup(FlushType::None);
+  return popupFrame && popupFrame->IsOpen();
+}
+
+bool XULButtonElement::IsOnMenu() const {
+  auto* popup = XULPopupElement::FromNodeOrNull(GetMenuParent());
+  return popup && popup->IsMenu();
+}
+
+bool XULButtonElement::IsOnMenuList() const {
+  if (XULMenuParentElement* menu = GetMenuParent()) {
+    return menu->GetParent() &&
+           menu->GetParent()->IsXULElement(nsGkAtoms::menulist);
+  }
+  return false;
+}
+
+bool XULButtonElement::IsOnMenuBar() const {
+  if (XULMenuParentElement* menu = GetMenuParent()) {
+    return menu->IsMenuBar();
+  }
+  return false;
+}
+
+nsMenuPopupFrame* XULButtonElement::GetContainingPopupWithoutFlushing() const {
+  if (XULPopupElement* popup = GetContainingPopupElement()) {
+    return do_QueryFrame(popup->GetPrimaryFrame());
+  }
+  return nullptr;
+}
+
+XULPopupElement* XULButtonElement::GetContainingPopupElement() const {
+  return XULPopupElement::FromNodeOrNull(GetMenuParent());
+}
+
+bool XULButtonElement::IsOnContextMenu() const {
+  if (nsMenuPopupFrame* popup = GetContainingPopupWithoutFlushing()) {
+    return popup->IsContextMenu();
+  }
+  return false;
+}
+
+void XULButtonElement::ToggleMenuState() {
+  if (IsMenuPopupOpen()) {
+    CloseMenuPopup(false);
+  } else {
+    OpenMenuPopup(false);
+  }
+}
+
+void XULButtonElement::KillMenuOpenTimer() {
+  if (mMenuOpenTimer) {
+    mMenuOpenTimer->Cancel();
+    mMenuOpenTimer = nullptr;
+  }
+}
+
+void XULButtonElement::OpenMenuPopup(bool aSelectFirstItem) {
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (!pm) {
+    return;
+  }
+
+  pm->KillMenuTimer();
+  if (!pm->MayShowMenu(this)) {
+    return;
+  }
+
+  if (RefPtr<XULMenuParentElement> parent = GetMenuParent()) {
+    parent->SetActiveMenuChild(this);
+  }
+
+  // Open the menu asynchronously.
+  OwnerDoc()->Dispatch(NS_NewRunnableFunction(
+      "AsyncOpenMenu", [self = RefPtr{this}, aSelectFirstItem] {
+        if (self->GetMenuParent() && !self->IsMenuActive()) {
+          return;
+        }
+        if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+          pm->ShowMenu(self, aSelectFirstItem);
+        }
+      }));
+}
+
+void XULButtonElement::CloseMenuPopup(bool aDeselectMenu) {
+  gMenuJustOpenedOrClosedTime = TimeStamp::Now();
+  // Close the menu asynchronously
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (!pm) {
+    return;
+  }
+  if (auto* popup = GetMenuPopupContent()) {
+    HidePopupOptions options{HidePopupOption::Async};
+    if (aDeselectMenu) {
+      options += HidePopupOption::DeselectMenu;
+    }
+    pm->HidePopup(popup, options);
+  }
+}
+
+int32_t XULButtonElement::MenuOpenCloseDelay() const {
+  if (IsOnMenuBar()) {
+    return 0;
+  }
+  return LookAndFeel::GetInt(LookAndFeel::IntID::SubmenuDelay, 300);  // ms
+}
+
+void XULButtonElement::ExecuteMenu(Modifiers aModifiers, int16_t aButton,
+                                   bool aIsTrusted) {
+  MOZ_ASSERT(IsMenu());
+
+  StopBlinking();
+
+  auto menuType = GetMenuType();
+  if (NS_WARN_IF(!menuType)) {
+    return;
+  }
+
+  // Because the command event is firing asynchronously, a flag is needed to
+  // indicate whether user input is being handled. This ensures that a popup
+  // window won't get blocked.
+  const bool userinput = dom::UserActivation::IsHandlingUserInput();
+
+  // Flip "checked" state if we're a checkbox menu, or an un-checked radio menu.
+  bool needToFlipChecked = false;
+  if (*menuType == MenuType::Checkbox ||
+      (*menuType == MenuType::Radio && !GetXULBoolAttr(nsGkAtoms::checked))) {
+    needToFlipChecked = !AttrValueIs(kNameSpaceID_None, nsGkAtoms::autocheck,
+                                     nsGkAtoms::_false, eCaseMatters);
+  }
+
+  mDelayedMenuCommandEvent = new nsXULMenuCommandEvent(
+      this, aIsTrusted, aModifiers, userinput, needToFlipChecked, aButton);
+  StartBlinking();
+}
+
+void XULButtonElement::StopBlinking() {
+  if (mMenuBlinkTimer) {
+    if (auto* parent = GetMenuParent()) {
+      parent->LockMenuUntilClosed(false);
+    }
+    mMenuBlinkTimer->Cancel();
+    mMenuBlinkTimer = nullptr;
+  }
+  mDelayedMenuCommandEvent = nullptr;
+}
+
+void XULButtonElement::PassMenuCommandEventToPopupManager() {
+  if (mDelayedMenuCommandEvent) {
+    if (RefPtr<nsXULPopupManager> pm = nsXULPopupManager::GetInstance()) {
+      RefPtr<nsXULMenuCommandEvent> event = std::move(mDelayedMenuCommandEvent);
+      nsCOMPtr<nsIContent> content = this;
+      pm->ExecuteMenu(content, event);
+    }
+  }
+  mDelayedMenuCommandEvent = nullptr;
+}
+
+static constexpr int32_t kBlinkDelay = 67;  // milliseconds
+
+void XULButtonElement::StartBlinking() {
+  if (!LookAndFeel::GetInt(LookAndFeel::IntID::ChosenMenuItemsShouldBlink)) {
+    PassMenuCommandEventToPopupManager();
+    return;
+  }
+
+  // Blink off.
+  UnsetAttr(kNameSpaceID_None, nsGkAtoms::menuactive, true);
+  if (auto* parent = GetMenuParent()) {
+    // Make this menu ignore events from now on.
+    parent->LockMenuUntilClosed(true);
+  }
+
+  // Set up a timer to blink back on.
+  NS_NewTimerWithFuncCallback(
+      getter_AddRefs(mMenuBlinkTimer),
+      [](nsITimer*, void* aClosure) MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+        RefPtr self = static_cast<XULButtonElement*>(aClosure);
+        if (auto* parent = self->GetMenuParent()) {
+          if (parent->GetActiveMenuChild() == self) {
+            // Restore the highlighting if we're still the active item.
+            self->SetAttr(kNameSpaceID_None, nsGkAtoms::menuactive, u"true"_ns,
+                          true);
+          }
+        }
+        // Reuse our timer to actually execute.
+        self->mMenuBlinkTimer->InitWithNamedFuncCallback(
+            [](nsITimer*, void* aClosure) MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+              RefPtr self = static_cast<XULButtonElement*>(aClosure);
+              if (auto* parent = self->GetMenuParent()) {
+                parent->LockMenuUntilClosed(false);
+              }
+              self->PassMenuCommandEventToPopupManager();
+              self->StopBlinking();
+            },
+            aClosure, kBlinkDelay, nsITimer::TYPE_ONE_SHOT,
+            "XULButtonElement::ContinueBlinking");
+      },
+      this, kBlinkDelay, nsITimer::TYPE_ONE_SHOT,
+      "XULButtonElement::StartBlinking", GetMainThreadSerialEventTarget());
+}
+
+void XULButtonElement::UnbindFromTree(bool aNullParent) {
+  StopBlinking();
+  nsXULElement::UnbindFromTree(aNullParent);
+}
+
+void XULButtonElement::ExecuteMenu(WidgetEvent& aEvent) {
+  MOZ_ASSERT(IsMenu());
+  if (nsCOMPtr<nsISound> sound = do_GetService("@mozilla.org/sound;1")) {
+    sound->PlayEventSound(nsISound::EVENT_MENU_EXECUTE);
+  }
+
+  Modifiers modifiers = 0;
+  if (WidgetInputEvent* inputEvent = aEvent.AsInputEvent()) {
+    modifiers = inputEvent->mModifiers;
+  }
+
+  int16_t button = 0;
+  if (WidgetMouseEventBase* mouseEvent = aEvent.AsMouseEventBase()) {
+    button = mouseEvent->mButton;
+  }
+
+  ExecuteMenu(modifiers, button, aEvent.IsTrusted());
+}
+
+void XULButtonElement::PostHandleEventForMenus(
+    EventChainPostVisitor& aVisitor) {
+  auto* event = aVisitor.mEvent;
+
+  if (event->mOriginalTarget != this) {
+    return;
+  }
+
+  if (auto* parent = GetMenuParent()) {
+    if (NS_WARN_IF(parent->IsLocked())) {
+      return;
+    }
+  }
+
+  // If a menu just opened, ignore the mouseup event that might occur after a
+  // the mousedown event that opened it. However, if a different mousedown event
+  // occurs, just clear this flag.
+  if (!gMenuJustOpenedOrClosedTime.IsNull()) {
+    if (event->mMessage == eMouseDown) {
+      gMenuJustOpenedOrClosedTime = TimeStamp();
+    } else if (event->mMessage == eMouseUp) {
+      return;
+    }
+  }
+
+  if (event->mMessage == eKeyPress && !IsDisabled()) {
+    WidgetKeyboardEvent* keyEvent = event->AsKeyboardEvent();
+    uint32_t keyCode = keyEvent->mKeyCode;
+#ifdef XP_MACOSX
+    // On mac, open menulist on either up/down arrow or space (w/o Cmd pressed)
+    if (!IsMenuPopupOpen() &&
+        ((keyEvent->mCharCode == ' ' && !keyEvent->IsMeta()) ||
+         (keyCode == NS_VK_UP || keyCode == NS_VK_DOWN))) {
+      // When pressing space, don't open the menu if performing an incremental
+      // search.
+      if (keyEvent->mCharCode != ' ' ||
+          !nsMenuPopupFrame::IsWithinIncrementalTime(keyEvent->mTimeStamp)) {
+        aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+        OpenMenuPopup(false);
+      }
+    }
+#else
+    // On other platforms, toggle menulist on unmodified F4 or Alt arrow
+    if ((keyCode == NS_VK_F4 && !keyEvent->IsAlt()) ||
+        ((keyCode == NS_VK_UP || keyCode == NS_VK_DOWN) && keyEvent->IsAlt())) {
+      aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+      ToggleMenuState();
+    }
+#endif
+  } else if (event->mMessage == eMouseDown &&
+             event->AsMouseEvent()->mButton == MouseButton::ePrimary &&
+#ifdef XP_MACOSX
+             // On mac, ctrl-click will send a context menu event from the
+             // widget, so we don't want to bring up the menu.
+             !event->AsMouseEvent()->IsControl() &&
+#endif
+             !IsDisabled() && !IsMenuItem()) {
+    // The menu item was selected. Bring up the menu.
+    // We have children.
+    // Don't prevent the default action here, since that will also cancel
+    // potential drag starts.
+    if (!IsOnMenu()) {
+      ToggleMenuState();
+    } else if (!IsMenuPopupOpen()) {
+      OpenMenuPopup(false);
+    }
+  } else if (event->mMessage == eMouseUp && IsMenuItem() && !IsDisabled() &&
+             !event->mFlags.mMultipleActionsPrevented) {
+    // We accept left and middle clicks on all menu items to activate the item.
+    // On context menus we also accept right click to activate the item, because
+    // right-clicking on an item in a context menu cannot open another context
+    // menu.
+    bool isMacCtrlClick = false;
+#ifdef XP_MACOSX
+    isMacCtrlClick = event->AsMouseEvent()->mButton == MouseButton::ePrimary &&
+                     event->AsMouseEvent()->IsControl();
+#endif
+    bool clickMightOpenContextMenu =
+        event->AsMouseEvent()->mButton == MouseButton::eSecondary ||
+        isMacCtrlClick;
+    if (!clickMightOpenContextMenu || IsOnContextMenu()) {
+      aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+      ExecuteMenu(*event);
+    }
+  } else if (event->mMessage == eContextMenu && IsOnContextMenu() &&
+             !IsMenuItem() && !IsDisabled()) {
+    // Make sure we cancel default processing of the context menu event so
+    // that it doesn't bubble and get seen again by the popuplistener and show
+    // another context menu.
+    aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+  } else if (event->mMessage == eMouseOut) {
+    KillMenuOpenTimer();
+    if (RefPtr<XULMenuParentElement> parent = GetMenuParent()) {
+      if (parent->GetActiveMenuChild() == this) {
+        // Deactivate the menu on mouse out in some cases...
+        const bool shouldDeactivate = [&] {
+          if (IsMenuPopupOpen()) {
+            // If we're open we never deselect. PopupClosed will do as needed.
+            return false;
+          }
+          if (auto* menubar = XULMenuBarElement::FromNode(*parent)) {
+            // De-select when exiting a menubar item, if the menubar wasn't
+            // activated by keyboard.
+            return !menubar->IsActiveByKeyboard();
+          }
+          if (IsOnMenuList()) {
+            // Don't de-select if on a menu-list. That matches Chromium and our
+            // historical Windows behavior, see bug 1197913.
+            return false;
+          }
+          // De-select elsewhere.
+          return true;
+        }();
+
+        if (shouldDeactivate) {
+          parent->SetActiveMenuChild(nullptr);
+        }
+      }
+    }
+  } else if (event->mMessage == eMouseMove && (IsOnMenu() || IsOnMenuBar())) {
+    // Use a tolerance to address situations where a user might perform a
+    // "wiggly" click that is accompanied by near-simultaneous mousemove events.
+    const TimeDuration kTolerance = TimeDuration::FromMilliseconds(200);
+    if (!gMenuJustOpenedOrClosedTime.IsNull() &&
+        gMenuJustOpenedOrClosedTime + kTolerance < TimeStamp::Now()) {
+      gMenuJustOpenedOrClosedTime = TimeStamp();
+      return;
+    }
+
+    if (IsDisabled() && IsOnMenuList()) {
+      return;
+    }
+
+    RefPtr<XULMenuParentElement> parent = GetMenuParent();
+    MOZ_ASSERT(parent, "How did IsOnMenu{,Bar} return true then?");
+
+    const bool isOnOpenMenubar =
+        parent->IsMenuBar() && parent->GetActiveMenuChild() &&
+        parent->GetActiveMenuChild()->IsMenuPopupOpen();
+
+    parent->SetActiveMenuChild(this);
+
+    // We need to check if we really became the current menu item or not.
+    if (!IsMenuActive()) {
+      // We didn't (presumably because a context menu was active)
+      return;
+    }
+    if (IsDisabled() || IsMenuItem() || IsMenuPopupOpen() || mMenuOpenTimer) {
+      // Disabled, or already opening or what not.
+      return;
+    }
+
+    if (parent->IsMenuBar() && !isOnOpenMenubar) {
+      // We should only open on hover in the menubar iff the menubar is open
+      // already.
+      return;
+    }
+
+    // A timer is used so that it doesn't open if the user moves the mouse
+    // quickly past the menu. The MenuOpenCloseDelay ensures that only menus
+    // have this behaviour.
+    NS_NewTimerWithFuncCallback(
+        getter_AddRefs(mMenuOpenTimer),
+        [](nsITimer*, void* aClosure) MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+          RefPtr self = static_cast<XULButtonElement*>(aClosure);
+          self->mMenuOpenTimer = nullptr;
+          if (self->IsMenuPopupOpen()) {
+            return;
+          }
+          // make sure we didn't open a context menu in the meantime
+          // (i.e. the user right-clicked while hovering over a submenu).
+          nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+          if (!pm) {
+            return;
+          }
+          if (pm->HasContextMenu(nullptr) && !self->IsOnContextMenu()) {
+            return;
+          }
+          if (!self->IsMenuActive()) {
+            return;
+          }
+          self->OpenMenuPopup(false);
+        },
+        this, MenuOpenCloseDelay(), nsITimer::TYPE_ONE_SHOT,
+        "XULButtonElement::OpenMenu", GetMainThreadSerialEventTarget());
+  }
+}
+
+nsresult XULButtonElement::PostHandleEvent(EventChainPostVisitor& aVisitor) {
+  if (aVisitor.mEventStatus == nsEventStatus_eConsumeNoDefault) {
+    return nsXULElement::PostHandleEvent(aVisitor);
+  }
+
+  if (IsMenu()) {
+    PostHandleEventForMenus(aVisitor);
+    return nsXULElement::PostHandleEvent(aVisitor);
+  }
+
+  auto* event = aVisitor.mEvent;
+  switch (event->mMessage) {
+    case eBlur: {
+      Blurred();
+      break;
+    }
+    case eKeyDown: {
+      WidgetKeyboardEvent* keyEvent = event->AsKeyboardEvent();
+      if (!keyEvent) {
+        break;
+      }
+      if (keyEvent->ShouldWorkAsSpaceKey() && aVisitor.mPresContext) {
+        EventStateManager* esm = aVisitor.mPresContext->EventStateManager();
+        // :hover:active state
+        esm->SetContentState(this, ElementState::HOVER);
+        esm->SetContentState(this, ElementState::ACTIVE);
+        mIsHandlingKeyEvent = true;
+      }
+      break;
+    }
+
+// On mac, Return fires the default button, not the focused one.
+#ifndef XP_MACOSX
+    case eKeyPress: {
+      WidgetKeyboardEvent* keyEvent = event->AsKeyboardEvent();
+      if (!keyEvent) {
+        break;
+      }
+      if (NS_VK_RETURN == keyEvent->mKeyCode) {
+        if (RefPtr<nsIDOMXULButtonElement> button = AsXULButton()) {
+          if (MouseClicked(*keyEvent)) {
+            aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+          }
+        }
+      }
+      break;
+    }
+#endif
+
+    case eKeyUp: {
+      WidgetKeyboardEvent* keyEvent = event->AsKeyboardEvent();
+      if (!keyEvent) {
+        break;
+      }
+      if (keyEvent->ShouldWorkAsSpaceKey()) {
+        mIsHandlingKeyEvent = false;
+        ElementState buttonState = State();
+        if (buttonState.HasAllStates(ElementState::ACTIVE |
+                                     ElementState::HOVER) &&
+            aVisitor.mPresContext) {
+          // return to normal state
+          EventStateManager* esm = aVisitor.mPresContext->EventStateManager();
+          esm->SetContentState(nullptr, ElementState::ACTIVE);
+          esm->SetContentState(nullptr, ElementState::HOVER);
+          if (MouseClicked(*keyEvent)) {
+            aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+          }
+        }
+      }
+      break;
+    }
+
+    case eMouseClick: {
+      WidgetMouseEvent* mouseEvent = event->AsMouseEvent();
+      if (mouseEvent->IsLeftClickEvent()) {
+        if (MouseClicked(*mouseEvent)) {
+          aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+        }
+      }
+      break;
+    }
+
+    default:
+      break;
+  }
+
+  return nsXULElement::PostHandleEvent(aVisitor);
+}
+
+void XULButtonElement::Blurred() {
+  ElementState buttonState = State();
+  if (mIsHandlingKeyEvent &&
+      buttonState.HasAllStates(ElementState::ACTIVE | ElementState::HOVER)) {
+    // Return to normal state
+    if (nsPresContext* pc = OwnerDoc()->GetPresContext()) {
+      EventStateManager* esm = pc->EventStateManager();
+      esm->SetContentState(nullptr, ElementState::ACTIVE);
+      esm->SetContentState(nullptr, ElementState::HOVER);
+    }
+  }
+  mIsHandlingKeyEvent = false;
+}
+
+bool XULButtonElement::MouseClicked(WidgetGUIEvent& aEvent) {
+  // Don't execute if we're disabled.
+  if (IsDisabled() || !IsInComposedDoc()) {
+    return false;
+  }
+
+  // Have the content handle the event, propagating it according to normal DOM
+  // rules.
+  RefPtr<mozilla::PresShell> presShell = OwnerDoc()->GetPresShell();
+  if (!presShell) {
+    return false;
+  }
+
+  // Execute the oncommand event handler.
+  WidgetInputEvent* inputEvent = aEvent.AsInputEvent();
+  WidgetMouseEventBase* mouseEvent = aEvent.AsMouseEventBase();
+  WidgetKeyboardEvent* keyEvent = aEvent.AsKeyboardEvent();
+  // TODO: Set aSourceEvent?
+  nsContentUtils::DispatchXULCommand(
+      this, aEvent.IsTrusted(), /* aSourceEvent = */ nullptr, presShell,
+      inputEvent->IsControl(), inputEvent->IsAlt(), inputEvent->IsShift(),
+      inputEvent->IsMeta(),
+      mouseEvent ? mouseEvent->mInputSource
+                 : (keyEvent ? MouseEvent_Binding::MOZ_SOURCE_KEYBOARD
+                             : MouseEvent_Binding::MOZ_SOURCE_UNKNOWN),
+      mouseEvent ? mouseEvent->mButton : 0);
+  return true;
+}
+
+bool XULButtonElement::IsMenu() const {
+  if (mIsAlwaysMenu) {
+    return true;
+  }
+  return IsAnyOfXULElements(nsGkAtoms::button, nsGkAtoms::toolbarbutton) &&
+         AttrValueIs(kNameSpaceID_None, nsGkAtoms::type, nsGkAtoms::menu,
+                     eCaseMatters);
+}
+
+void XULButtonElement::UncheckRadioSiblings() {
+  MOZ_ASSERT(!nsContentUtils::IsSafeToRunScript());
+  MOZ_ASSERT(GetMenuType() == Some(MenuType::Radio));
+  nsAutoString groupName;
+  GetAttr(nsGkAtoms::name, groupName);
+
+  nsIContent* parent = GetParent();
+  if (!parent) {
+    return;
+  }
+
+  auto ShouldUncheck = [&](const nsIContent& aSibling) {
+    const auto* button = XULButtonElement::FromNode(aSibling);
+    if (!button || button->GetMenuType() != Some(MenuType::Radio)) {
+      return false;
+    }
+    if (const auto* attr = button->GetParsedAttr(nsGkAtoms::name)) {
+      if (!attr->Equals(groupName, eCaseMatters)) {
+        return false;
+      }
+    } else if (!groupName.IsEmpty()) {
+      return false;
+    }
+    // we're in the same group, only uncheck if we're checked (for some reason,
+    // some tests rely on that specifically).
+    return button->GetXULBoolAttr(nsGkAtoms::checked);
+  };
+
+  for (nsIContent* child = parent->GetFirstChild(); child;
+       child = child->GetNextSibling()) {
+    if (child == this || !ShouldUncheck(*child)) {
+      continue;
+    }
+    child->AsElement()->UnsetAttr(nsGkAtoms::checked, IgnoreErrors());
+  }
+}
+
+void XULButtonElement::AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                                    const nsAttrValue* aValue,
+                                    const nsAttrValue* aOldValue,
+                                    nsIPrincipal* aSubjectPrincipal,
+                                    bool aNotify) {
+  nsXULElement::AfterSetAttr(aNamespaceID, aName, aValue, aOldValue,
+                             aSubjectPrincipal, aNotify);
+  if (IsAlwaysMenu() && aNamespaceID == kNameSpaceID_None) {
+    // We need to uncheck radio siblings when we're a checked radio and switch
+    // groups, or become checked.
+    const bool shouldUncheckSiblings = [&] {
+      if (aName == nsGkAtoms::type || aName == nsGkAtoms::name) {
+        return *GetMenuType() == MenuType::Radio &&
+               GetXULBoolAttr(nsGkAtoms::checked);
+      }
+      if (aName == nsGkAtoms::checked && aValue &&
+          aValue->Equals(nsGkAtoms::_true, eCaseMatters)) {
+        return *GetMenuType() == MenuType::Radio;
+      }
+      return false;
+    }();
+    if (shouldUncheckSiblings) {
+      UncheckRadioSiblings();
+    }
+  }
+}
+
+auto XULButtonElement::GetMenuType() const -> Maybe<MenuType> {
+  if (!IsAlwaysMenu()) {
+    return Nothing();
+  }
+
+  static Element::AttrValuesArray values[] = {nsGkAtoms::checkbox,
+                                              nsGkAtoms::radio, nullptr};
+  switch (FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::type, values,
+                          eCaseMatters)) {
+    case 0:
+      return Some(MenuType::Checkbox);
+    case 1:
+      return Some(MenuType::Radio);
+    default:
+      return Some(MenuType::Normal);
+  }
+}
+
+XULMenuBarElement* XULButtonElement::GetMenuBar() const {
+  if (!IsMenu()) {
+    return nullptr;
+  }
+  return FirstAncestorOfType<XULMenuBarElement>();
+}
+
+XULMenuParentElement* XULButtonElement::GetMenuParent() const {
+  if (IsXULElement(nsGkAtoms::menulist)) {
+    return nullptr;
+  }
+  return FirstAncestorOfType<XULMenuParentElement>();
+}
+
+XULPopupElement* XULButtonElement::GetMenuPopupContent() const {
+  if (!IsMenu()) {
+    return nullptr;
+  }
+  for (auto* child = GetFirstChild(); child; child = child->GetNextSibling()) {
+    if (auto* popup = XULPopupElement::FromNode(child)) {
+      return popup;
+    }
+  }
+  return nullptr;
+}
+
+nsMenuPopupFrame* XULButtonElement::GetMenuPopupWithoutFlushing() const {
+  return const_cast<XULButtonElement*>(this)->GetMenuPopup(FlushType::None);
+}
+
+nsMenuPopupFrame* XULButtonElement::GetMenuPopup(FlushType aFlushType) {
+  RefPtr popup = GetMenuPopupContent();
+  if (!popup) {
+    return nullptr;
+  }
+  return do_QueryFrame(popup->GetPrimaryFrame(aFlushType));
+}
+
+bool XULButtonElement::OpenedWithKey() const {
+  auto* menubar = GetMenuBar();
+  return menubar && menubar->IsActiveByKeyboard();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULButtonElement.h b/dom/xul/XULButtonElement.h
new file mode 100644
index 0000000000..fc880210a2
--- /dev/null
+++ b/dom/xul/XULButtonElement.h
@@ -0,0 +1,123 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef dom_xul_XULButtonElement_h__
+#define dom_xul_XULButtonElement_h__
+
+#include "mozilla/Attributes.h"
+#include "nsINode.h"
+#include "nsXULElement.h"
+
+class nsMenuBarFrame;
+class nsMenuPopupFrame;
+class nsXULMenuCommandEvent;
+
+namespace mozilla::dom {
+
+class KeyboardEvent;
+class XULPopupElement;
+class XULMenuBarElement;
+class XULMenuParentElement;
+
+class XULButtonElement : public nsXULElement {
+ public:
+  explicit XULButtonElement(
+      already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+  ~XULButtonElement() override;
+
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY bool MouseClicked(WidgetGUIEvent&);
+  MOZ_CAN_RUN_SCRIPT nsresult PostHandleEvent(EventChainPostVisitor&) override;
+  MOZ_CAN_RUN_SCRIPT void PostHandleEventForMenus(EventChainPostVisitor&);
+  MOZ_CAN_RUN_SCRIPT void HandleEnterKeyPress(WidgetEvent&);
+
+  void PopupOpened();
+  MOZ_CAN_RUN_SCRIPT void PopupClosed(bool aDeselectMenu);
+
+  XULPopupElement* GetContainingPopupElement() const;
+  nsMenuPopupFrame* GetContainingPopupWithoutFlushing() const;
+  MOZ_CAN_RUN_SCRIPT void ToggleMenuState();
+  bool IsMenuPopupOpen();
+
+  bool IsMenuItem() const { return NodeInfo()->Equals(nsGkAtoms::menuitem); }
+  bool IsMenuList() const { return NodeInfo()->Equals(nsGkAtoms::menulist); }
+  bool IsMenuActive() const;
+  MOZ_CAN_RUN_SCRIPT void OpenMenuPopup(bool aSelectFirstItem);
+  void CloseMenuPopup(bool aDeselectMenu);
+
+  bool IsOnMenu() const;
+  bool IsOnMenuList() const;
+  bool IsOnMenuBar() const;
+  bool IsOnContextMenu() const;
+
+  XULMenuParentElement* GetMenuParent() const;
+
+  void UnbindFromTree(bool aNullParent) override;
+
+  MOZ_CAN_RUN_SCRIPT bool HandleKeyPress(KeyboardEvent& keyEvent);
+  bool OpenedWithKey() const;
+  // Called to execute our command handler.
+  MOZ_CAN_RUN_SCRIPT void ExecuteMenu(WidgetEvent&);
+  MOZ_CAN_RUN_SCRIPT void ExecuteMenu(Modifiers, int16_t aButton,
+                                      bool aIsTrusted);
+
+  // Whether we are a menu/menulist/menuitem element.
+  bool IsAlwaysMenu() const { return mIsAlwaysMenu; }
+  // Whether we should behave like a menu. This is the above plus buttons with
+  // type=menu attribute.
+  bool IsMenu() const;
+
+  nsChangeHint GetAttributeChangeHint(const nsAtom* aAttribute,
+                                      int32_t aModType) const override;
+  void AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                    const nsAttrValue* aValue, const nsAttrValue* aOldValue,
+                    nsIPrincipal* aSubjectPrincipal, bool aNotify) override;
+
+  NS_IMPL_FROMNODE_HELPER(XULButtonElement,
+                          IsAnyOfXULElements(nsGkAtoms::checkbox,
+                                             nsGkAtoms::radio, nsGkAtoms::thumb,
+                                             nsGkAtoms::button, nsGkAtoms::menu,
+                                             nsGkAtoms::menulist,
+                                             nsGkAtoms::menuitem,
+                                             nsGkAtoms::toolbarbutton,
+                                             nsGkAtoms::toolbarpaletteitem,
+                                             nsGkAtoms::scrollbarbutton))
+
+  nsMenuPopupFrame* GetMenuPopup(FlushType aFlushType);
+  nsMenuPopupFrame* GetMenuPopupWithoutFlushing() const;
+  XULPopupElement* GetMenuPopupContent() const;
+  int32_t MenuOpenCloseDelay() const;
+
+  bool IsDisabled() const { return GetXULBoolAttr(nsGkAtoms::disabled); }
+
+ private:
+  XULMenuBarElement* GetMenuBar() const;
+  void Blurred();
+  enum class MenuType {
+    Checkbox,
+    Radio,
+    Normal,
+  };
+  Maybe<MenuType> GetMenuType() const;
+
+  void UncheckRadioSiblings();
+  void StopBlinking();
+  MOZ_CAN_RUN_SCRIPT void StartBlinking();
+  void KillMenuOpenTimer();
+  MOZ_CAN_RUN_SCRIPT void PassMenuCommandEventToPopupManager();
+
+  bool mIsHandlingKeyEvent = false;
+
+  // Whether this is a XULMenuElement.
+  const bool mIsAlwaysMenu;
+  RefPtr<nsXULMenuCommandEvent> mDelayedMenuCommandEvent;
+  nsCOMPtr<nsITimer> mMenuOpenTimer;
+  nsCOMPtr<nsITimer> mMenuBlinkTimer;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/xul/XULFrameElement.cpp b/dom/xul/XULFrameElement.cpp
new file mode 100644
index 0000000000..efbeb15829
--- /dev/null
+++ b/dom/xul/XULFrameElement.cpp
@@ -0,0 +1,200 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsCOMPtr.h"
+#include "nsIBrowser.h"
+#include "nsIContent.h"
+#include "nsIOpenWindowInfo.h"
+#include "nsFrameLoader.h"
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/dom/HTMLIFrameElement.h"
+#include "mozilla/dom/WindowProxyHolder.h"
+#include "mozilla/dom/XULFrameElement.h"
+#include "mozilla/dom/XULFrameElementBinding.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(XULFrameElement)
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(XULFrameElement, nsXULElement)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mFrameLoader)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOpenWindowInfo)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(XULFrameElement, nsXULElement)
+  if (tmp->mFrameLoader) {
+    tmp->mFrameLoader->Destroy();
+  }
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mOpenWindowInfo)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(XULFrameElement, nsXULElement,
+                                             nsFrameLoaderOwner)
+
+JSObject* XULFrameElement::WrapNode(JSContext* aCx,
+                                    JS::Handle<JSObject*> aGivenProto) {
+  return XULFrameElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+nsDocShell* XULFrameElement::GetDocShell() {
+  RefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
+  return frameLoader ? frameLoader->GetDocShell(IgnoreErrors()) : nullptr;
+}
+
+already_AddRefed<nsIWebNavigation> XULFrameElement::GetWebNavigation() {
+  nsCOMPtr<nsIDocShell> docShell = GetDocShell();
+  nsCOMPtr<nsIWebNavigation> webnav = do_QueryInterface(docShell);
+  return webnav.forget();
+}
+
+Nullable<WindowProxyHolder> XULFrameElement::GetContentWindow() {
+  RefPtr<nsDocShell> docShell = GetDocShell();
+  if (docShell) {
+    return docShell->GetWindowProxy();
+  }
+
+  return nullptr;
+}
+
+Document* XULFrameElement::GetContentDocument() {
+  nsCOMPtr<nsIDocShell> docShell = GetDocShell();
+  if (docShell) {
+    nsCOMPtr<nsPIDOMWindowOuter> win = docShell->GetWindow();
+    if (win) {
+      return win->GetDoc();
+    }
+  }
+  return nullptr;
+}
+
+uint64_t XULFrameElement::BrowserId() {
+  if (mFrameLoader) {
+    if (auto* bc = mFrameLoader->GetExtantBrowsingContext()) {
+      return bc->GetBrowserId();
+    }
+  }
+  return 0;
+}
+
+nsIOpenWindowInfo* XULFrameElement::GetOpenWindowInfo() const {
+  return mOpenWindowInfo;
+}
+
+void XULFrameElement::SetOpenWindowInfo(nsIOpenWindowInfo* aInfo) {
+  mOpenWindowInfo = aInfo;
+}
+
+void XULFrameElement::LoadSrc() {
+  if (!IsInUncomposedDoc() || !OwnerDoc()->GetRootElement()) {
+    return;
+  }
+  RefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
+  if (!frameLoader) {
+    // We may have had a nsIOpenWindowInfo set on us by browser chrome, due to
+    // being used as the target for a `window.open` call. Fetch that information
+    // if it's available, and clear it out so we don't read it again.
+    nsCOMPtr<nsIOpenWindowInfo> openWindowInfo = mOpenWindowInfo.forget();
+
+    // false as the networkCreated parameter so that xul:iframe/browser/editor
+    // session history handling works like dynamic html:iframes. Usually xul
+    // elements are used in chrome, which doesn't have session history at all.
+    mFrameLoader = nsFrameLoader::Create(this, false, openWindowInfo);
+    if (NS_WARN_IF(!mFrameLoader)) {
+      return;
+    }
+
+    AsyncEventDispatcher::RunDOMEventWhenSafe(
+        *this, u"XULFrameLoaderCreated"_ns, CanBubble::eYes);
+  }
+
+  mFrameLoader->LoadFrame(false);
+}
+
+void XULFrameElement::SwapFrameLoaders(HTMLIFrameElement& aOtherLoaderOwner,
+                                       ErrorResult& rv) {
+  aOtherLoaderOwner.SwapFrameLoaders(this, rv);
+}
+
+void XULFrameElement::SwapFrameLoaders(XULFrameElement& aOtherLoaderOwner,
+                                       ErrorResult& rv) {
+  if (&aOtherLoaderOwner == this) {
+    // nothing to do
+    return;
+  }
+
+  aOtherLoaderOwner.SwapFrameLoaders(this, rv);
+}
+
+void XULFrameElement::SwapFrameLoaders(nsFrameLoaderOwner* aOtherLoaderOwner,
+                                       mozilla::ErrorResult& rv) {
+  if (RefPtr<Document> doc = GetComposedDoc()) {
+    // SwapWithOtherLoader relies on frames being up-to-date.
+    doc->FlushPendingNotifications(FlushType::Frames);
+  }
+
+  RefPtr<nsFrameLoader> loader = GetFrameLoader();
+  RefPtr<nsFrameLoader> otherLoader = aOtherLoaderOwner->GetFrameLoader();
+  if (!loader || !otherLoader) {
+    rv.Throw(NS_ERROR_NOT_IMPLEMENTED);
+    return;
+  }
+
+  rv = loader->SwapWithOtherLoader(otherLoader, this, aOtherLoaderOwner);
+}
+
+nsresult XULFrameElement::BindToTree(BindContext& aContext, nsINode& aParent) {
+  nsresult rv = nsXULElement::BindToTree(aContext, aParent);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (IsInUncomposedDoc()) {
+    NS_ASSERTION(!nsContentUtils::IsSafeToRunScript(),
+                 "Missing a script blocker!");
+    // We're in a document now.  Kick off the frame load.
+    LoadSrc();
+  }
+
+  return NS_OK;
+}
+
+void XULFrameElement::UnbindFromTree(bool aNullParent) {
+  if (RefPtr<nsFrameLoader> frameLoader = GetFrameLoader()) {
+    frameLoader->Destroy();
+  }
+  mFrameLoader = nullptr;
+
+  nsXULElement::UnbindFromTree(aNullParent);
+}
+
+void XULFrameElement::DestroyContent() {
+  RefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
+  if (frameLoader) {
+    frameLoader->Destroy();
+  }
+  mFrameLoader = nullptr;
+
+  nsXULElement::DestroyContent();
+}
+
+void XULFrameElement::AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                                   const nsAttrValue* aValue,
+                                   const nsAttrValue* aOldValue,
+                                   nsIPrincipal* aSubjectPrincipal,
+                                   bool aNotify) {
+  if (aNamespaceID == kNameSpaceID_None) {
+    if (aName == nsGkAtoms::src && aValue) {
+      LoadSrc();
+    } else if (aName == nsGkAtoms::disablefullscreen && mFrameLoader) {
+      if (auto* bc = mFrameLoader->GetExtantBrowsingContext()) {
+        MOZ_ALWAYS_SUCCEEDS(bc->SetFullscreenAllowedByOwner(!aValue));
+      }
+    }
+  }
+
+  return nsXULElement::AfterSetAttr(aNamespaceID, aName, aValue, aOldValue,
+                                    aSubjectPrincipal, aNotify);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULFrameElement.h b/dom/xul/XULFrameElement.h
new file mode 100644
index 0000000000..13a65f42e8
--- /dev/null
+++ b/dom/xul/XULFrameElement.h
@@ -0,0 +1,84 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULFrameElement_h__
+#define XULFrameElement_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/Nullable.h"
+#include "mozilla/dom/WindowProxyHolder.h"
+#include "js/TypeDecls.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIOpenWindowInfo.h"
+#include "nsWrapperCache.h"
+#include "nsString.h"
+#include "nsXULElement.h"
+#include "nsFrameLoaderOwner.h"
+
+class nsIWebNavigation;
+class nsFrameLoader;
+
+namespace mozilla {
+class ErrorResult;
+
+namespace dom {
+
+class BrowsingContext;
+
+class XULFrameElement final : public nsXULElement, public nsFrameLoaderOwner {
+ public:
+  explicit XULFrameElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+      : nsXULElement(std::move(aNodeInfo)) {}
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(XULFrameElement, nsXULElement)
+
+  // XULFrameElement.webidl
+  nsDocShell* GetDocShell();
+  already_AddRefed<nsIWebNavigation> GetWebNavigation();
+  Nullable<WindowProxyHolder> GetContentWindow();
+  Document* GetContentDocument();
+  uint64_t BrowserId();
+  nsIOpenWindowInfo* GetOpenWindowInfo() const;
+  void SetOpenWindowInfo(nsIOpenWindowInfo* aInfo);
+
+  void SwapFrameLoaders(mozilla::dom::HTMLIFrameElement& aOtherLoaderOwner,
+                        mozilla::ErrorResult& rv);
+  void SwapFrameLoaders(XULFrameElement& aOtherLoaderOwner,
+                        mozilla::ErrorResult& rv);
+  void SwapFrameLoaders(nsFrameLoaderOwner* aOtherLoaderOwner,
+                        mozilla::ErrorResult& rv);
+
+  // nsIContent
+  nsresult BindToTree(BindContext&, nsINode& aParent) override;
+  void UnbindFromTree(bool aNullParent) override;
+  void DestroyContent() override;
+
+  void AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                    const nsAttrValue* aValue, const nsAttrValue* aOldValue,
+                    nsIPrincipal* aSubjectPrincipal, bool aNotify) override;
+
+  NS_IMPL_FROMNODE_HELPER(XULFrameElement,
+                          IsAnyOfXULElements(nsGkAtoms::iframe,
+                                             nsGkAtoms::browser,
+                                             nsGkAtoms::editor))
+
+ protected:
+  virtual ~XULFrameElement() = default;
+
+  JSObject* WrapNode(JSContext* aCx,
+                     JS::Handle<JSObject*> aGivenProto) override;
+
+  void LoadSrc();
+
+ private:
+  nsCOMPtr<nsIOpenWindowInfo> mOpenWindowInfo;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // XULFrameElement_h
diff --git a/dom/xul/XULMenuBarElement.cpp b/dom/xul/XULMenuBarElement.cpp
new file mode 100644
index 0000000000..f32698b3cd
--- /dev/null
+++ b/dom/xul/XULMenuBarElement.cpp
@@ -0,0 +1,103 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "XULMenuBarElement.h"
+#include "MenuBarListener.h"
+#include "XULButtonElement.h"
+#include "nsXULPopupManager.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/dom/BindContext.h"
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/Try.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(XULMenuBarElement)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(XULMenuBarElement,
+                                                XULMenuParentElement)
+  if (tmp->mListener) {
+    tmp->mListener->Detach();
+    tmp->mListener = nullptr;
+  }
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(XULMenuBarElement,
+                                                  XULMenuParentElement)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mListener)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(XULMenuBarElement,
+                                               XULMenuParentElement)
+
+XULMenuBarElement::XULMenuBarElement(
+    already_AddRefed<class NodeInfo>&& aNodeInfo)
+    : XULMenuParentElement(std::move(aNodeInfo)) {}
+
+XULMenuBarElement::~XULMenuBarElement() { MOZ_DIAGNOSTIC_ASSERT(!mListener); }
+
+void XULMenuBarElement::SetActive(bool aActiveFlag) {
+  // If the activity is not changed, there is nothing to do.
+  if (mIsActive == aActiveFlag) {
+    return;
+  }
+
+  // We can't activate a menubar outside of the document.
+  if (!IsInComposedDoc()) {
+    MOZ_ASSERT(!mIsActive, "How?");
+    return;
+  }
+
+  if (!aActiveFlag) {
+    // If there is a request to deactivate the menu bar, check to see whether
+    // there is a menu popup open for the menu bar. In this case, don't
+    // deactivate the menu bar.
+    if (auto* activeChild = GetActiveMenuChild()) {
+      if (activeChild->IsMenuPopupOpen()) {
+        return;
+      }
+    }
+  }
+
+  mIsActive = aActiveFlag;
+  if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+    pm->SetActiveMenuBar(this, aActiveFlag);
+  }
+  if (!aActiveFlag) {
+    mActiveByKeyboard = false;
+    SetActiveMenuChild(nullptr);
+  }
+
+  RefPtr dispatcher = new AsyncEventDispatcher(
+      this, aActiveFlag ? u"DOMMenuBarActive"_ns : u"DOMMenuBarInactive"_ns,
+      CanBubble::eYes, ChromeOnlyDispatch::eNo);
+  DebugOnly<nsresult> rv = dispatcher->PostDOMEvent();
+  NS_ASSERTION(NS_SUCCEEDED(rv), "AsyncEventDispatcher failed to dispatch");
+}
+
+nsresult XULMenuBarElement::BindToTree(BindContext& aContext,
+                                       nsINode& aParent) {
+  MOZ_TRY(XULMenuParentElement::BindToTree(aContext, aParent));
+  MOZ_DIAGNOSTIC_ASSERT(!mListener);
+  if (aContext.InComposedDoc()) {
+    mListener = new MenuBarListener(*this);
+  }
+  return NS_OK;
+}
+
+void XULMenuBarElement::UnbindFromTree(bool aNullParent) {
+  if (mListener) {
+    mListener->Detach();
+    mListener = nullptr;
+  }
+  if (NS_WARN_IF(mIsActive)) {
+    // Clean up silently when getting removed from the document while active.
+    mIsActive = false;
+    if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+      pm->SetActiveMenuBar(this, false);
+    }
+  }
+  return XULMenuParentElement::UnbindFromTree(aNullParent);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULMenuBarElement.h b/dom/xul/XULMenuBarElement.h
new file mode 100644
index 0000000000..117682642d
--- /dev/null
+++ b/dom/xul/XULMenuBarElement.h
@@ -0,0 +1,59 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULMenuBarElement_h__
+#define XULMenuBarElement_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/NameSpaceConstants.h"
+#include "nsINode.h"
+#include "nsISupports.h"
+#include "XULMenuParentElement.h"
+
+namespace mozilla::dom {
+
+class KeyboardEvent;
+class XULButtonElement;
+class MenuBarListener;
+
+nsXULElement* NS_NewXULMenuBarElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+class XULMenuBarElement final : public XULMenuParentElement {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(XULMenuBarElement,
+                                           XULMenuParentElement)
+  NS_IMPL_FROMNODE_WITH_TAG(XULMenuBarElement, kNameSpaceID_XUL, menubar)
+
+  explicit XULMenuBarElement(already_AddRefed<class NodeInfo>&&);
+
+  MOZ_CAN_RUN_SCRIPT void SetActive(bool);
+  bool IsActive() const { return mIsActive; }
+
+  void SetActiveByKeyboard() { mActiveByKeyboard = true; }
+  bool IsActiveByKeyboard() const { return mActiveByKeyboard; }
+
+  nsresult BindToTree(BindContext&, nsINode& aParent) override;
+  void UnbindFromTree(bool aNullParent) override;
+
+ protected:
+  ~XULMenuBarElement() override;
+
+  // Whether or not the menu bar is active (a menu item is highlighted or
+  // shown).
+  bool mIsActive = false;
+
+  // Whether the menubar was made active via the keyboard.
+  bool mActiveByKeyboard = false;
+
+  // The event listener that listens to document key presses and so on.
+  RefPtr<MenuBarListener> mListener;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // XULMenuBarElement_h
diff --git a/dom/xul/XULMenuElement.cpp b/dom/xul/XULMenuElement.cpp
new file mode 100644
index 0000000000..928e850012
--- /dev/null
+++ b/dom/xul/XULMenuElement.cpp
@@ -0,0 +1,82 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/dom/XULMenuElement.h"
+#include "mozilla/StaticAnalysisFunctions.h"
+#include "mozilla/dom/XULButtonElement.h"
+#include "mozilla/dom/XULMenuElementBinding.h"
+#include "mozilla/dom/XULPopupElement.h"
+#include "mozilla/dom/KeyboardEvent.h"
+#include "mozilla/dom/KeyboardEventBinding.h"
+#include "nsXULPopupManager.h"
+#include "nsMenuPopupFrame.h"
+
+namespace mozilla::dom {
+
+JSObject* XULMenuElement::WrapNode(JSContext* aCx,
+                                   JS::Handle<JSObject*> aGivenProto) {
+  return XULMenuElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+Element* XULMenuElement::GetActiveMenuChild() {
+  RefPtr popup = GetMenuPopupContent();
+  return popup ? popup->GetActiveMenuChild() : nullptr;
+}
+
+void XULMenuElement::SetActiveMenuChild(Element* aChild) {
+  RefPtr popup = GetMenuPopupContent();
+  if (NS_WARN_IF(!popup)) {
+    return;
+  }
+
+  if (!aChild) {
+    popup->SetActiveMenuChild(nullptr);
+    return;
+  }
+  auto* button = XULButtonElement::FromNode(aChild);
+  if (NS_WARN_IF(!button) || NS_WARN_IF(!button->IsMenu())) {
+    return;
+  }
+  // KnownLive because it's aChild.
+  popup->SetActiveMenuChild(MOZ_KnownLive(button));
+}
+
+bool XULButtonElement::HandleKeyPress(KeyboardEvent& keyEvent) {
+  RefPtr<nsXULPopupManager> pm = nsXULPopupManager::GetInstance();
+  if (!pm) {
+    return false;
+  }
+
+  // if event has already been handled, bail
+  if (keyEvent.DefaultPrevented()) {
+    return false;
+  }
+
+  if (keyEvent.IsMenuAccessKeyPressed()) {
+    return false;
+  }
+
+  nsMenuPopupFrame* popupFrame = GetMenuPopup(FlushType::Frames);
+  if (NS_WARN_IF(!popupFrame)) {
+    return false;
+  }
+
+  uint32_t keyCode = keyEvent.KeyCode();
+  switch (keyCode) {
+    case KeyboardEvent_Binding::DOM_VK_UP:
+    case KeyboardEvent_Binding::DOM_VK_DOWN:
+    case KeyboardEvent_Binding::DOM_VK_HOME:
+    case KeyboardEvent_Binding::DOM_VK_END: {
+      nsNavigationDirection theDirection;
+      theDirection = NS_DIRECTION_FROM_KEY_CODE(popupFrame, keyCode);
+      return pm->HandleKeyboardNavigationInPopup(popupFrame, theDirection);
+    }
+    default:
+      return pm->HandleShortcutNavigation(keyEvent, popupFrame);
+  }
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULMenuElement.h b/dom/xul/XULMenuElement.h
new file mode 100644
index 0000000000..5def6839fc
--- /dev/null
+++ b/dom/xul/XULMenuElement.h
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_dom_XULMenuElement_h
+#define mozilla_dom_XULMenuElement_h
+
+#include "XULButtonElement.h"
+#include "nsINode.h"
+#include "nsXULElement.h"
+
+namespace mozilla::dom {
+
+class KeyboardEvent;
+
+class XULMenuElement final : public XULButtonElement {
+ public:
+  explicit XULMenuElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+      : XULButtonElement(std::move(aNodeInfo)) {}
+
+  MOZ_CAN_RUN_SCRIPT void SetActiveMenuChild(Element*);
+  Element* GetActiveMenuChild();
+
+  NS_IMPL_FROMNODE_HELPER(XULMenuElement,
+                          IsAnyOfXULElements(nsGkAtoms::menu,
+                                             nsGkAtoms::menulist));
+
+ private:
+  virtual ~XULMenuElement() = default;
+  JSObject* WrapNode(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) final;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // XULMenuElement_h
diff --git a/dom/xul/XULMenuParentElement.cpp b/dom/xul/XULMenuParentElement.cpp
new file mode 100644
index 0000000000..c457187025
--- /dev/null
+++ b/dom/xul/XULMenuParentElement.cpp
@@ -0,0 +1,398 @@
+/* -*- Mode: C++; tab-width: 4; 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 "XULMenuParentElement.h"
+#include "XULButtonElement.h"
+#include "XULMenuBarElement.h"
+#include "XULPopupElement.h"
+#include "mozilla/LookAndFeel.h"
+#include "mozilla/StaticAnalysisFunctions.h"
+#include "mozilla/TextEvents.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/dom/KeyboardEvent.h"
+#include "mozilla/EventDispatcher.h"
+#include "nsDebug.h"
+#include "nsMenuPopupFrame.h"
+#include "nsString.h"
+#include "nsStringFwd.h"
+#include "nsUTF8Utils.h"
+#include "nsXULElement.h"
+#include "nsXULPopupManager.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(XULMenuParentElement,
+                                               nsXULElement)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(XULMenuParentElement, nsXULElement,
+                                   mActiveItem)
+
+XULMenuParentElement::XULMenuParentElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+    : nsXULElement(std::move(aNodeInfo)) {}
+
+XULMenuParentElement::~XULMenuParentElement() = default;
+
+class MenuActivateEvent final : public Runnable {
+ public:
+  MenuActivateEvent(Element* aMenu, bool aIsActivate)
+      : Runnable("MenuActivateEvent"), mMenu(aMenu), mIsActivate(aIsActivate) {}
+
+  // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230, bug 1535398)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHOD Run() override {
+    nsAutoString domEventToFire;
+    if (mIsActivate) {
+      // Highlight the menu.
+      mMenu->SetAttr(kNameSpaceID_None, nsGkAtoms::menuactive, u"true"_ns,
+                     true);
+      // The menuactivated event is used by accessibility to track the user's
+      // movements through menus
+      domEventToFire.AssignLiteral("DOMMenuItemActive");
+    } else {
+      // Unhighlight the menu.
+      mMenu->UnsetAttr(kNameSpaceID_None, nsGkAtoms::menuactive, true);
+      domEventToFire.AssignLiteral("DOMMenuItemInactive");
+    }
+
+    RefPtr<nsPresContext> pc = mMenu->OwnerDoc()->GetPresContext();
+    RefPtr<dom::Event> event = NS_NewDOMEvent(mMenu, pc, nullptr);
+    event->InitEvent(domEventToFire, true, true);
+
+    event->SetTrusted(true);
+
+    EventDispatcher::DispatchDOMEvent(mMenu, nullptr, event, pc, nullptr);
+    return NS_OK;
+  }
+
+ private:
+  const RefPtr<Element> mMenu;
+  bool mIsActivate;
+};
+
+static void ActivateOrDeactivate(XULButtonElement& aButton, bool aActivate) {
+  if (!aButton.IsMenu()) {
+    return;
+  }
+
+  if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+    if (aActivate) {
+      // Cancel the close timer if selecting a menu within the popup to be
+      // closed.
+      pm->CancelMenuTimer(aButton.GetContainingPopupWithoutFlushing());
+    } else if (auto* popup = aButton.GetMenuPopupWithoutFlushing()) {
+      if (popup->IsOpen()) {
+        // Set up the close timer if deselecting an open sub-menu.
+        pm->HidePopupAfterDelay(popup, aButton.MenuOpenCloseDelay());
+      }
+    }
+  }
+
+  nsCOMPtr<nsIRunnable> event = new MenuActivateEvent(&aButton, aActivate);
+  aButton.OwnerDoc()->Dispatch(event.forget());
+}
+
+XULButtonElement* XULMenuParentElement::GetContainingMenu() const {
+  if (IsMenuBar()) {
+    return nullptr;
+  }
+  auto* button = XULButtonElement::FromNodeOrNull(GetParent());
+  if (!button || !button->IsMenu()) {
+    return nullptr;
+  }
+  return button;
+}
+
+void XULMenuParentElement::LockMenuUntilClosed(bool aLock) {
+  if (IsMenuBar()) {
+    return;
+  }
+  mLocked = aLock;
+  // Lock/Unlock the parent menu too.
+  if (XULButtonElement* menu = GetContainingMenu()) {
+    if (XULMenuParentElement* parent = menu->GetMenuParent()) {
+      parent->LockMenuUntilClosed(aLock);
+    }
+  }
+}
+
+void XULMenuParentElement::SetActiveMenuChild(XULButtonElement* aChild,
+                                              ByKey aByKey) {
+  if (aChild == mActiveItem) {
+    return;
+  }
+
+  if (mActiveItem) {
+    ActivateOrDeactivate(*mActiveItem, false);
+  }
+  mActiveItem = nullptr;
+
+  if (auto* menuBar = XULMenuBarElement::FromNode(*this)) {
+    // KnownLive because `this` is known-live by definition.
+    MOZ_KnownLive(menuBar)->SetActive(!!aChild);
+  }
+
+  if (!aChild) {
+    return;
+  }
+
+  // When a menu opens a submenu, the mouse will often be moved onto a sibling
+  // before moving onto an item within the submenu, causing the parent to become
+  // deselected. We need to ensure that the parent menu is reselected when an
+  // item in the submenu is selected.
+  if (RefPtr menu = GetContainingMenu()) {
+    if (RefPtr parent = menu->GetMenuParent()) {
+      parent->SetActiveMenuChild(menu, aByKey);
+    }
+  }
+
+  mActiveItem = aChild;
+  ActivateOrDeactivate(*mActiveItem, true);
+
+  if (IsInMenuList()) {
+    if (nsMenuPopupFrame* f = do_QueryFrame(GetPrimaryFrame())) {
+      f->EnsureActiveMenuListItemIsVisible();
+#ifdef XP_WIN
+      // On Windows, a menulist should update its value whenever navigation was
+      // done by the keyboard.
+      //
+      // NOTE(emilio): This is a rather odd per-platform behavior difference,
+      // but other browsers also do this.
+      if (aByKey == ByKey::Yes && f->IsOpen()) {
+        // Fire a command event as the new item, but we don't want to close the
+        // menu, blink it, or update any other state of the menuitem. The
+        // command event will cause the item to be selected.
+        RefPtr<mozilla::PresShell> presShell = OwnerDoc()->GetPresShell();
+        nsContentUtils::DispatchXULCommand(aChild, /* aTrusted = */ true,
+                                           nullptr, presShell, false, false,
+                                           false, false);
+      }
+#endif
+    }
+  }
+}
+
+static bool IsValidMenuItem(const XULMenuParentElement& aMenuParent,
+                            const nsIContent& aContent) {
+  const auto* button = XULButtonElement::FromNode(aContent);
+  if (!button || !button->IsMenu()) {
+    return false;
+  }
+  if (!button->GetPrimaryFrame()) {
+    // Hidden buttons are not focusable/activatable.
+    return false;
+  }
+  if (!button->IsDisabled()) {
+    return true;
+  }
+  // In the menubar or a menulist disabled items are always skipped.
+  const bool skipDisabled =
+      LookAndFeel::GetInt(LookAndFeel::IntID::SkipNavigatingDisabledMenuItem) ||
+      aMenuParent.IsMenuBar() || aMenuParent.IsInMenuList();
+  return !skipDisabled;
+}
+
+enum class StartKind { Parent, Item };
+
+template <bool aForward>
+static XULButtonElement* DoGetNextMenuItem(
+    const XULMenuParentElement& aMenuParent, const nsIContent& aStart,
+    StartKind aStartKind) {
+  nsIContent* start =
+      aStartKind == StartKind::Item
+          ? (aForward ? aStart.GetNextSibling() : aStart.GetPreviousSibling())
+          : (aForward ? aStart.GetFirstChild() : aStart.GetLastChild());
+  for (nsIContent* node = start; node;
+       node = aForward ? node->GetNextSibling() : node->GetPreviousSibling()) {
+    if (IsValidMenuItem(aMenuParent, *node)) {
+      return static_cast<XULButtonElement*>(node);
+    }
+    if (node->IsXULElement(nsGkAtoms::menugroup)) {
+      if (XULButtonElement* child = DoGetNextMenuItem<aForward>(
+              aMenuParent, *node, StartKind::Parent)) {
+        return child;
+      }
+    }
+  }
+  if (aStartKind == StartKind::Item && aStart.GetParent() &&
+      aStart.GetParent()->IsXULElement(nsGkAtoms::menugroup)) {
+    // We haven't found anything in aStart's sibling list, but if we're in a
+    // group we need to keep looking.
+    return DoGetNextMenuItem<aForward>(aMenuParent, *aStart.GetParent(),
+                                       StartKind::Item);
+  }
+  return nullptr;
+}
+
+XULButtonElement* XULMenuParentElement::GetFirstMenuItem() const {
+  return DoGetNextMenuItem<true>(*this, *this, StartKind::Parent);
+}
+
+XULButtonElement* XULMenuParentElement::GetLastMenuItem() const {
+  return DoGetNextMenuItem<false>(*this, *this, StartKind::Parent);
+}
+
+XULButtonElement* XULMenuParentElement::GetNextMenuItemFrom(
+    const XULButtonElement& aStartingItem) const {
+  return DoGetNextMenuItem<true>(*this, aStartingItem, StartKind::Item);
+}
+
+XULButtonElement* XULMenuParentElement::GetPrevMenuItemFrom(
+    const XULButtonElement& aStartingItem) const {
+  return DoGetNextMenuItem<false>(*this, aStartingItem, StartKind::Item);
+}
+
+XULButtonElement* XULMenuParentElement::GetNextMenuItem(Wrap aWrap) const {
+  if (mActiveItem) {
+    if (auto* next = GetNextMenuItemFrom(*mActiveItem)) {
+      return next;
+    }
+    if (aWrap == Wrap::No) {
+      return nullptr;
+    }
+  }
+  return GetFirstMenuItem();
+}
+
+XULButtonElement* XULMenuParentElement::GetPrevMenuItem(Wrap aWrap) const {
+  if (mActiveItem) {
+    if (auto* prev = GetPrevMenuItemFrom(*mActiveItem)) {
+      return prev;
+    }
+    if (aWrap == Wrap::No) {
+      return nullptr;
+    }
+  }
+  return GetLastMenuItem();
+}
+
+void XULMenuParentElement::SelectFirstItem() {
+  if (RefPtr firstItem = GetFirstMenuItem()) {
+    SetActiveMenuChild(firstItem);
+  }
+}
+
+XULButtonElement* XULMenuParentElement::FindMenuWithShortcut(
+    KeyboardEvent& aKeyEvent) const {
+  using AccessKeyArray = AutoTArray<uint32_t, 10>;
+  AccessKeyArray accessKeys;
+  WidgetKeyboardEvent* nativeKeyEvent =
+      aKeyEvent.WidgetEventPtr()->AsKeyboardEvent();
+  if (nativeKeyEvent) {
+    nativeKeyEvent->GetAccessKeyCandidates(accessKeys);
+  }
+  const uint32_t charCode = aKeyEvent.CharCode();
+  if (accessKeys.IsEmpty() && charCode) {
+    accessKeys.AppendElement(charCode);
+  }
+  if (accessKeys.IsEmpty()) {
+    return nullptr;  // no character was pressed so just return
+  }
+  XULButtonElement* foundMenu = nullptr;
+  size_t foundIndex = AccessKeyArray::NoIndex;
+  for (auto* item = GetFirstMenuItem(); item;
+       item = GetNextMenuItemFrom(*item)) {
+    nsAutoString shortcutKey;
+    item->GetAttr(nsGkAtoms::accesskey, shortcutKey);
+    if (shortcutKey.IsEmpty()) {
+      continue;
+    }
+
+    ToLowerCase(shortcutKey);
+    const char16_t* start = shortcutKey.BeginReading();
+    const char16_t* end = shortcutKey.EndReading();
+    uint32_t ch = UTF16CharEnumerator::NextChar(&start, end);
+    size_t index = accessKeys.IndexOf(ch);
+    if (index == AccessKeyArray::NoIndex) {
+      continue;
+    }
+    if (foundIndex == AccessKeyArray::NoIndex || index < foundIndex) {
+      foundMenu = item;
+      foundIndex = index;
+    }
+  }
+  return foundMenu;
+}
+
+XULButtonElement* XULMenuParentElement::FindMenuWithShortcut(
+    const nsAString& aString, bool& aDoAction) const {
+  aDoAction = false;
+  uint32_t accessKeyMatchCount = 0;
+  uint32_t matchCount = 0;
+
+  XULButtonElement* foundAccessKeyMenu = nullptr;
+  XULButtonElement* foundMenuBeforeCurrent = nullptr;
+  XULButtonElement* foundMenuAfterCurrent = nullptr;
+
+  bool foundActive = false;
+  for (auto* item = GetFirstMenuItem(); item;
+       item = GetNextMenuItemFrom(*item)) {
+    nsAutoString textKey;
+    // Get the shortcut attribute.
+    item->GetAttr(nsGkAtoms::accesskey, textKey);
+    const bool isAccessKey = !textKey.IsEmpty();
+    if (textKey.IsEmpty()) {  // No shortcut, try first letter
+      item->GetAttr(nsGkAtoms::label, textKey);
+      if (textKey.IsEmpty()) {  // No label, try another attribute (value)
+        item->GetAttr(nsGkAtoms::value, textKey);
+      }
+    }
+
+    const bool isActive = item == GetActiveMenuChild();
+    foundActive |= isActive;
+
+    if (!StringBeginsWith(
+            nsContentUtils::TrimWhitespace<
+                nsContentUtils::IsHTMLWhitespaceOrNBSP>(textKey, false),
+            aString, nsCaseInsensitiveStringComparator)) {
+      continue;
+    }
+    // There is one match
+    matchCount++;
+    if (isAccessKey) {
+      // There is one shortcut-key match
+      accessKeyMatchCount++;
+      // Record the matched item. If there is only one matched shortcut
+      // item, do it
+      foundAccessKeyMenu = item;
+    }
+    // Get the active status
+    if (isActive && aString.Length() > 1 && !foundMenuBeforeCurrent) {
+      // If there is more than one char typed and the current item matches, the
+      // current item has highest priority, otherwise the item next to current
+      // has highest priority.
+      return item;
+    }
+    if (!foundActive || isActive) {
+      // It's a first candidate item located before/on the current item
+      if (!foundMenuBeforeCurrent) {
+        foundMenuBeforeCurrent = item;
+      }
+    } else {
+      if (!foundMenuAfterCurrent) {
+        foundMenuAfterCurrent = item;
+      }
+    }
+  }
+
+  aDoAction = !IsInMenuList() && (matchCount == 1 || accessKeyMatchCount == 1);
+
+  if (accessKeyMatchCount == 1) {
+    // We have one matched accesskey item
+    return foundAccessKeyMenu;
+  }
+  // If we have matched an item after the current, use it.
+  if (foundMenuAfterCurrent) {
+    return foundMenuAfterCurrent;
+  }
+  // If we haven't, use the item before the current, if any.
+  return foundMenuBeforeCurrent;
+}
+
+void XULMenuParentElement::HandleEnterKeyPress(WidgetEvent& aEvent) {
+  if (RefPtr child = GetActiveMenuChild()) {
+    child->HandleEnterKeyPress(aEvent);
+  }
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULMenuParentElement.h b/dom/xul/XULMenuParentElement.h
new file mode 100644
index 0000000000..263ec058c1
--- /dev/null
+++ b/dom/xul/XULMenuParentElement.h
@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULMenuParentElement_h__
+#define XULMenuParentElement_h__
+
+#include "mozilla/Attributes.h"
+#include "nsISupports.h"
+#include "nsXULElement.h"
+
+namespace mozilla::dom {
+
+class KeyboardEvent;
+class XULButtonElement;
+
+nsXULElement* NS_NewXULMenuParentElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+class XULMenuParentElement : public nsXULElement {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(XULMenuParentElement, nsXULElement)
+
+  explicit XULMenuParentElement(
+      already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+  bool IsMenuBar() const { return NodeInfo()->Equals(nsGkAtoms::menubar); }
+  bool IsMenu() const { return !IsMenuBar(); }
+
+  bool IsLocked() const { return mLocked; }
+
+  void LockMenuUntilClosed(bool aLock);
+
+  bool IsInMenuList() const {
+    return GetParent() && GetParent()->IsXULElement(nsGkAtoms::menulist);
+  }
+
+  XULButtonElement* FindMenuWithShortcut(KeyboardEvent&) const;
+  XULButtonElement* FindMenuWithShortcut(const nsAString& aString,
+                                         bool& aDoAction) const;
+  MOZ_CAN_RUN_SCRIPT void HandleEnterKeyPress(WidgetEvent&);
+
+  NS_IMPL_FROMNODE_HELPER(XULMenuParentElement,
+                          IsAnyOfXULElements(nsGkAtoms::menupopup,
+                                             nsGkAtoms::panel,
+                                             nsGkAtoms::tooltip,
+                                             nsGkAtoms::menubar));
+
+  XULButtonElement* GetActiveMenuChild() const { return mActiveItem.get(); }
+  enum class ByKey : bool { No, Yes };
+  MOZ_CAN_RUN_SCRIPT void SetActiveMenuChild(XULButtonElement*,
+                                             ByKey = ByKey::No);
+
+  XULButtonElement* GetFirstMenuItem() const;
+  XULButtonElement* GetLastMenuItem() const;
+
+  XULButtonElement* GetNextMenuItemFrom(const XULButtonElement&) const;
+  XULButtonElement* GetPrevMenuItemFrom(const XULButtonElement&) const;
+
+  enum class Wrap : bool { No, Yes };
+  XULButtonElement* GetNextMenuItem(Wrap = Wrap::Yes) const;
+  XULButtonElement* GetPrevMenuItem(Wrap = Wrap::Yes) const;
+
+  XULButtonElement* GetContainingMenu() const;
+
+  MOZ_CAN_RUN_SCRIPT void SelectFirstItem();
+
+ protected:
+  RefPtr<XULButtonElement> mActiveItem;
+  bool mLocked = false;
+
+  ~XULMenuParentElement() override;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // XULMenuParentElement_h
diff --git a/dom/xul/XULPersist.cpp b/dom/xul/XULPersist.cpp
new file mode 100644
index 0000000000..5048bd3ddd
--- /dev/null
+++ b/dom/xul/XULPersist.cpp
@@ -0,0 +1,247 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+#include "XULPersist.h"
+
+#include "nsIXULStore.h"
+#include "nsIStringEnumerator.h"
+#include "nsServiceManagerUtils.h"
+#include "mozilla/BasePrincipal.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "nsContentUtils.h"
+#include "nsIAppWindow.h"
+
+namespace mozilla::dom {
+
+static bool IsRootElement(Element* aElement) {
+  return aElement->OwnerDoc()->GetRootElement() == aElement;
+}
+
+// FIXME: This is a hack to differentiate "attribute is missing" from "attribute
+// is present but empty". Use a newline to avoid virtually all collisions.
+// Ideally the XUL store would be able to store this more reasonably.
+constexpr auto kMissingAttributeToken = u"-moz-missing\n"_ns;
+
+static bool ShouldPersistAttribute(Element* aElement, nsAtom* aAttribute) {
+  if (IsRootElement(aElement)) {
+    // This is not an element of the top document, its owner is
+    // not an AppWindow. Persist it.
+    if (aElement->OwnerDoc()->GetInProcessParentDocument()) {
+      return true;
+    }
+    // The following attributes of xul:window should be handled in
+    // AppWindow::SavePersistentAttributes instead of here.
+    if (aAttribute == nsGkAtoms::screenX || aAttribute == nsGkAtoms::screenY ||
+        aAttribute == nsGkAtoms::width || aAttribute == nsGkAtoms::height ||
+        aAttribute == nsGkAtoms::sizemode) {
+      return false;
+    }
+  }
+  return true;
+}
+
+NS_IMPL_ISUPPORTS(XULPersist, nsIDocumentObserver)
+
+XULPersist::XULPersist(Document* aDocument)
+    : nsStubDocumentObserver(), mDocument(aDocument) {}
+
+XULPersist::~XULPersist() = default;
+
+void XULPersist::Init() {
+  ApplyPersistentAttributes();
+  mDocument->AddObserver(this);
+}
+
+void XULPersist::DropDocumentReference() {
+  mDocument->RemoveObserver(this);
+  mDocument = nullptr;
+}
+
+void XULPersist::AttributeChanged(dom::Element* aElement, int32_t aNameSpaceID,
+                                  nsAtom* aAttribute, int32_t aModType,
+                                  const nsAttrValue* aOldValue) {
+  NS_ASSERTION(aElement->OwnerDoc() == mDocument, "unexpected doc");
+
+  if (aNameSpaceID != kNameSpaceID_None) {
+    return;
+  }
+
+  // See if there is anything we need to persist in the localstore.
+  nsAutoString persist;
+  // Persistence of attributes of xul:window is handled in AppWindow.
+  if (aElement->GetAttr(nsGkAtoms::persist, persist) &&
+      ShouldPersistAttribute(aElement, aAttribute) &&
+      // XXXldb This should check that it's a token, not just a substring.
+      persist.Find(nsDependentAtomString(aAttribute)) >= 0) {
+    // Might not need this, but be safe for now.
+    nsCOMPtr<nsIDocumentObserver> kungFuDeathGrip(this);
+    nsContentUtils::AddScriptRunner(NewRunnableMethod<Element*, nsAtom*>(
+        "dom::XULPersist::Persist", this, &XULPersist::Persist, aElement,
+        aAttribute));
+  }
+}
+
+void XULPersist::Persist(Element* aElement, nsAtom* aAttribute) {
+  if (!mDocument) {
+    return;
+  }
+  // For non-chrome documents, persistance is simply broken
+  if (!mDocument->NodePrincipal()->IsSystemPrincipal()) {
+    return;
+  }
+
+  if (!mLocalStore) {
+    mLocalStore = do_GetService("@mozilla.org/xul/xulstore;1");
+    if (NS_WARN_IF(!mLocalStore)) {
+      return;
+    }
+  }
+
+  nsAutoString id;
+
+  aElement->GetAttr(nsGkAtoms::id, id);
+  nsAtomString attrstr(aAttribute);
+
+  nsAutoCString utf8uri;
+  nsresult rv = mDocument->GetDocumentURI()->GetSpec(utf8uri);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return;
+  }
+
+  // Persisting attributes to top level windows is handled by AppWindow.
+  if (IsRootElement(aElement)) {
+    if (nsCOMPtr<nsIAppWindow> win =
+            mDocument->GetAppWindowIfToplevelChrome()) {
+      return;
+    }
+  }
+
+  NS_ConvertUTF8toUTF16 uri(utf8uri);
+  nsAutoString valuestr;
+  if (!aElement->GetAttr(aAttribute, valuestr)) {
+    valuestr = kMissingAttributeToken;
+  }
+
+  mLocalStore->SetValue(uri, id, attrstr, valuestr);
+  NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "value set");
+}
+
+nsresult XULPersist::ApplyPersistentAttributes() {
+  if (!mDocument) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  // For non-chrome documents, persistance is simply broken
+  if (!mDocument->NodePrincipal()->IsSystemPrincipal()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  // Add all of the 'persisted' attributes into the content
+  // model.
+  if (!mLocalStore) {
+    mLocalStore = do_GetService("@mozilla.org/xul/xulstore;1");
+    if (NS_WARN_IF(!mLocalStore)) {
+      return NS_ERROR_NOT_INITIALIZED;
+    }
+  }
+
+  nsCOMArray<Element> elements;
+
+  nsAutoCString utf8uri;
+  nsresult rv = mDocument->GetDocumentURI()->GetSpec(utf8uri);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+  NS_ConvertUTF8toUTF16 uri(utf8uri);
+
+  // Get a list of element IDs for which persisted values are available
+  nsCOMPtr<nsIStringEnumerator> ids;
+  rv = mLocalStore->GetIDsEnumerator(uri, getter_AddRefs(ids));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  bool hasmore;
+  while (NS_SUCCEEDED(ids->HasMore(&hasmore)) && hasmore) {
+    nsAutoString id;
+    ids->GetNext(id);
+
+    // We want to hold strong refs to the elements while applying
+    // persistent attributes, just in case.
+    const nsTArray<Element*>* allElements = mDocument->GetAllElementsForId(id);
+    if (!allElements) {
+      continue;
+    }
+    elements.Clear();
+    elements.SetCapacity(allElements->Length());
+    for (Element* element : *allElements) {
+      elements.AppendObject(element);
+    }
+
+    rv = ApplyPersistentAttributesToElements(id, uri, elements);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult XULPersist::ApplyPersistentAttributesToElements(
+    const nsAString& aID, const nsAString& aDocURI,
+    nsCOMArray<Element>& aElements) {
+  nsresult rv = NS_OK;
+  // Get a list of attributes for which persisted values are available
+  nsCOMPtr<nsIStringEnumerator> attrs;
+  rv = mLocalStore->GetAttributeEnumerator(aDocURI, aID, getter_AddRefs(attrs));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  bool hasmore;
+  while (NS_SUCCEEDED(attrs->HasMore(&hasmore)) && hasmore) {
+    nsAutoString attrstr;
+    attrs->GetNext(attrstr);
+
+    nsAutoString value;
+    rv = mLocalStore->GetValue(aDocURI, aID, attrstr, value);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    RefPtr<nsAtom> attr = NS_Atomize(attrstr);
+    if (NS_WARN_IF(!attr)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    uint32_t cnt = aElements.Length();
+    for (int32_t i = int32_t(cnt) - 1; i >= 0; --i) {
+      Element* element = aElements.SafeElementAt(i);
+      if (!element) {
+        continue;
+      }
+
+      // Applying persistent attributes to top level windows is handled
+      // by AppWindow.
+      if (IsRootElement(element)) {
+        if (nsCOMPtr<nsIAppWindow> win =
+                mDocument->GetAppWindowIfToplevelChrome()) {
+          continue;
+        }
+      }
+
+      if (value == kMissingAttributeToken) {
+        Unused << element->UnsetAttr(kNameSpaceID_None, attr, true);
+      } else {
+        Unused << element->SetAttr(kNameSpaceID_None, attr, value, true);
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULPersist.h b/dom/xul/XULPersist.h
new file mode 100644
index 0000000000..80cdac5dee
--- /dev/null
+++ b/dom/xul/XULPersist.h
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_dom_XULPersist_h
+#define mozilla_dom_XULPersist_h
+
+#include "nsStubDocumentObserver.h"
+
+#ifndef MOZ_NEW_XULSTORE
+#  include "nsCOMPtr.h"
+class nsIXULStore;
+#endif
+
+template <typename T>
+class nsCOMArray;
+
+namespace mozilla::dom {
+
+/**
+ * This class synchronizes element attributes (such as window sizing) with the
+ * live elements in a Document and with the XULStore. The XULStore persists
+ * these attributes to the file system. This class is created and owned by the
+ * Document and must only be created in the parent process. It only presists
+ * chrome document element attributes.
+ */
+class XULPersist final : public nsStubDocumentObserver {
+ public:
+  NS_DECL_ISUPPORTS
+
+  explicit XULPersist(Document* aDocument);
+  void Init();
+  void DropDocumentReference();
+
+  NS_DECL_NSIMUTATIONOBSERVER_ATTRIBUTECHANGED
+
+ protected:
+  void Persist(mozilla::dom::Element* aElement, nsAtom* aAttribute);
+
+ private:
+  ~XULPersist();
+  nsresult ApplyPersistentAttributes();
+  nsresult ApplyPersistentAttributesToElements(const nsAString& aID,
+                                               const nsAString& aDocURI,
+                                               nsCOMArray<Element>& aElements);
+
+  nsCOMPtr<nsIXULStore> mLocalStore;
+
+  // A weak pointer to our document. Nulled out by DropDocumentReference.
+  Document* MOZ_NON_OWNING_REF mDocument;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_XULPersist_h
diff --git a/dom/xul/XULPopupElement.cpp b/dom/xul/XULPopupElement.cpp
new file mode 100644
index 0000000000..d60173900e
--- /dev/null
+++ b/dom/xul/XULPopupElement.cpp
@@ -0,0 +1,339 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "XULMenuParentElement.h"
+#include "nsCOMPtr.h"
+#include "nsICSSDeclaration.h"
+#include "nsIContent.h"
+#include "nsNameSpaceManager.h"
+#include "nsGkAtoms.h"
+#include "nsMenuPopupFrame.h"
+#include "nsStringFwd.h"
+#include "nsView.h"
+#include "mozilla/AppUnits.h"
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/dom/DOMRect.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/XULPopupElement.h"
+#include "mozilla/dom/XULButtonElement.h"
+#include "mozilla/dom/XULMenuElement.h"
+#include "mozilla/dom/XULPopupElementBinding.h"
+#ifdef MOZ_WAYLAND
+#  include "mozilla/WidgetUtilsGtk.h"
+#endif
+
+namespace mozilla::dom {
+
+nsXULElement* NS_NewXULPopupElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo) {
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
+  auto* nim = nodeInfo->NodeInfoManager();
+  return new (nim) XULPopupElement(nodeInfo.forget());
+}
+
+JSObject* XULPopupElement::WrapNode(JSContext* aCx,
+                                    JS::Handle<JSObject*> aGivenProto) {
+  return XULPopupElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+nsMenuPopupFrame* XULPopupElement::GetFrame(FlushType aFlushType) {
+  nsIFrame* f = GetPrimaryFrame(aFlushType);
+  MOZ_ASSERT(!f || f->IsMenuPopupFrame());
+  return static_cast<nsMenuPopupFrame*>(f);
+}
+
+void XULPopupElement::OpenPopup(Element* aAnchorElement,
+                                const StringOrOpenPopupOptions& aOptions,
+                                int32_t aXPos, int32_t aYPos,
+                                bool aIsContextMenu, bool aAttributesOverride,
+                                Event* aTriggerEvent) {
+  nsAutoString position;
+  if (aOptions.IsOpenPopupOptions()) {
+    const OpenPopupOptions& options = aOptions.GetAsOpenPopupOptions();
+    position = options.mPosition;
+    aXPos = options.mX;
+    aYPos = options.mY;
+    aIsContextMenu = options.mIsContextMenu;
+    aAttributesOverride = options.mAttributesOverride;
+    aTriggerEvent = options.mTriggerEvent;
+  } else {
+    position = aOptions.GetAsString();
+  }
+
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (pm) {
+    // As a special case for popups that are menus when no anchor or position
+    // are specified, open the popup with ShowMenu instead of ShowPopup so that
+    // the popup is aligned with the menu.
+    if (!aAnchorElement && position.IsEmpty() && GetPrimaryFrame()) {
+      if (auto* menu = GetContainingMenu()) {
+        pm->ShowMenu(menu, false);
+        return;
+      }
+    }
+
+    pm->ShowPopup(this, aAnchorElement, position, aXPos, aYPos, aIsContextMenu,
+                  aAttributesOverride, false, aTriggerEvent);
+  }
+}
+
+void XULPopupElement::OpenPopupAtScreen(int32_t aXPos, int32_t aYPos,
+                                        bool aIsContextMenu,
+                                        Event* aTriggerEvent) {
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (pm) {
+    pm->ShowPopupAtScreen(this, aXPos, aYPos, aIsContextMenu, aTriggerEvent);
+  }
+}
+
+void XULPopupElement::OpenPopupAtScreenRect(const nsAString& aPosition,
+                                            int32_t aXPos, int32_t aYPos,
+                                            int32_t aWidth, int32_t aHeight,
+                                            bool aIsContextMenu,
+                                            bool aAttributesOverride,
+                                            Event* aTriggerEvent) {
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (pm) {
+    pm->ShowPopupAtScreenRect(
+        this, aPosition, nsIntRect(aXPos, aYPos, aWidth, aHeight),
+        aIsContextMenu, aAttributesOverride, aTriggerEvent);
+  }
+}
+
+void XULPopupElement::HidePopup(bool aCancel) {
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (!pm) {
+    return;
+  }
+  HidePopupOptions options{HidePopupOption::DeselectMenu};
+  if (aCancel) {
+    options += HidePopupOption::IsRollup;
+  }
+  pm->HidePopup(this, options);
+}
+
+static Modifiers ConvertModifiers(const ActivateMenuItemOptions& aModifiers) {
+  Modifiers modifiers = 0;
+  if (aModifiers.mCtrlKey) {
+    modifiers |= MODIFIER_CONTROL;
+  }
+  if (aModifiers.mAltKey) {
+    modifiers |= MODIFIER_ALT;
+  }
+  if (aModifiers.mShiftKey) {
+    modifiers |= MODIFIER_SHIFT;
+  }
+  if (aModifiers.mMetaKey) {
+    modifiers |= MODIFIER_META;
+  }
+  return modifiers;
+}
+
+void XULPopupElement::PopupOpened(bool aSelectFirstItem) {
+  if (aSelectFirstItem) {
+    SelectFirstItem();
+  }
+  if (RefPtr button = GetContainingMenu()) {
+    if (RefPtr parent = button->GetMenuParent()) {
+      parent->SetActiveMenuChild(button);
+    }
+  }
+}
+
+void XULPopupElement::PopupClosed(bool aDeselectMenu) {
+  LockMenuUntilClosed(false);
+  SetActiveMenuChild(nullptr);
+  auto dispatcher = MakeRefPtr<AsyncEventDispatcher>(
+      this, u"DOMMenuInactive"_ns, CanBubble::eYes, ChromeOnlyDispatch::eNo);
+  dispatcher->PostDOMEvent();
+  if (RefPtr button = GetContainingMenu()) {
+    button->PopupClosed(aDeselectMenu);
+  }
+}
+
+void XULPopupElement::ActivateItem(Element& aItemElement,
+                                   const ActivateMenuItemOptions& aOptions,
+                                   ErrorResult& aRv) {
+  if (!Contains(&aItemElement)) {
+    return aRv.ThrowInvalidStateError("Menu item is not inside this menu.");
+  }
+
+  Modifiers modifiers = ConvertModifiers(aOptions);
+
+  // First, check if a native menu is open, and activate the item in it.
+  if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+    if (pm->ActivateNativeMenuItem(&aItemElement, modifiers, aOptions.mButton,
+                                   aRv)) {
+      return;
+    }
+  }
+
+  auto* item = XULButtonElement::FromNode(aItemElement);
+  if (!item || !item->IsMenu()) {
+    return aRv.ThrowInvalidStateError("Not a menu item");
+  }
+
+  if (!item->GetPrimaryFrame(FlushType::Frames)) {
+    return aRv.ThrowInvalidStateError("Menu item is hidden");
+  }
+
+  auto* popup = item->GetContainingPopupElement();
+  if (!popup) {
+    return aRv.ThrowInvalidStateError("No popup");
+  }
+
+  nsMenuPopupFrame* frame = popup->GetFrame(FlushType::None);
+  if (!frame || !frame->IsOpen()) {
+    return aRv.ThrowInvalidStateError("Popup is not open");
+  }
+
+  // This is a chrome-only API, so we're trusted.
+  const bool trusted = true;
+  // KnownLive because item is aItemElement.
+  MOZ_KnownLive(item)->ExecuteMenu(modifiers, aOptions.mButton, trusted);
+}
+
+void XULPopupElement::MoveTo(int32_t aLeft, int32_t aTop) {
+  if (nsMenuPopupFrame* menuPopupFrame = do_QueryFrame(GetPrimaryFrame())) {
+    menuPopupFrame->MoveTo(CSSIntPoint(aLeft, aTop), true);
+  }
+}
+
+void XULPopupElement::MoveToAnchor(Element* aAnchorElement,
+                                   const nsAString& aPosition, int32_t aXPos,
+                                   int32_t aYPos, bool aAttributesOverride) {
+  nsMenuPopupFrame* menuPopupFrame = GetFrame(FlushType::None);
+  if (menuPopupFrame && menuPopupFrame->IsVisibleOrShowing()) {
+    menuPopupFrame->MoveToAnchor(aAnchorElement, aPosition, aXPos, aYPos,
+                                 aAttributesOverride);
+  }
+}
+
+void XULPopupElement::SizeTo(int32_t aWidth, int32_t aHeight) {
+  nsAutoCString width;
+  nsAutoCString height;
+  width.AppendInt(aWidth);
+  width.AppendLiteral("px");
+  height.AppendInt(aHeight);
+  height.AppendLiteral("px");
+
+  nsCOMPtr<nsICSSDeclaration> style = Style();
+  style->SetProperty("width"_ns, width, ""_ns, IgnoreErrors());
+  style->SetProperty("height"_ns, height, ""_ns, IgnoreErrors());
+
+  // If the popup is open, force a reposition of the popup after resizing it
+  // with notifications set to true so that the popuppositioned event is fired.
+  nsMenuPopupFrame* menuPopupFrame = do_QueryFrame(GetPrimaryFrame());
+  if (menuPopupFrame && menuPopupFrame->PopupState() == ePopupShown) {
+    menuPopupFrame->SetPopupPosition(false);
+  }
+}
+
+void XULPopupElement::GetState(nsString& aState) {
+  // set this here in case there's no frame for the popup
+  aState.AssignLiteral("closed");
+
+  if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+    switch (pm->GetPopupState(this)) {
+      case ePopupShown:
+        aState.AssignLiteral("open");
+        break;
+      case ePopupShowing:
+      case ePopupPositioning:
+      case ePopupOpening:
+      case ePopupVisible:
+        aState.AssignLiteral("showing");
+        break;
+      case ePopupHiding:
+      case ePopupInvisible:
+        aState.AssignLiteral("hiding");
+        break;
+      case ePopupClosed:
+        break;
+      default:
+        MOZ_ASSERT_UNREACHABLE("Bad popup state");
+        break;
+    }
+  }
+}
+
+nsINode* XULPopupElement::GetTriggerNode() const {
+  nsMenuPopupFrame* menuPopupFrame = do_QueryFrame(GetPrimaryFrame());
+  return nsMenuPopupFrame::GetTriggerContent(menuPopupFrame);
+}
+
+// FIXME(emilio): should probably be renamed to GetAnchorElement?
+Element* XULPopupElement::GetAnchorNode() const {
+  nsMenuPopupFrame* menuPopupFrame = do_QueryFrame(GetPrimaryFrame());
+  if (!menuPopupFrame) {
+    return nullptr;
+  }
+  return Element::FromNodeOrNull(menuPopupFrame->GetAnchor());
+}
+
+already_AddRefed<DOMRect> XULPopupElement::GetOuterScreenRect() {
+  RefPtr<DOMRect> rect = new DOMRect(ToSupports(OwnerDoc()));
+
+  // Return an empty rectangle if the popup is not open.
+  nsMenuPopupFrame* menuPopupFrame =
+      do_QueryFrame(GetPrimaryFrame(FlushType::Frames));
+  if (!menuPopupFrame || !menuPopupFrame->IsOpen()) {
+    return rect.forget();
+  }
+
+  Maybe<CSSRect> screenRect;
+
+  if (menuPopupFrame->IsNativeMenu()) {
+    // For native menus we can't query the true size. Use the anchor rect
+    // instead, which at least has the position at which we were intending to
+    // open the menu.
+    screenRect = Some(CSSRect(menuPopupFrame->GetScreenAnchorRect()));
+  } else {
+    // For non-native menus, query the bounds from the widget.
+    if (nsView* view = menuPopupFrame->GetView()) {
+      if (nsIWidget* widget = view->GetWidget()) {
+        screenRect = Some(widget->GetScreenBounds() /
+                          menuPopupFrame->PresContext()->CSSToDevPixelScale());
+      }
+    }
+  }
+
+  if (screenRect) {
+    rect->SetRect(screenRect->X(), screenRect->Y(), screenRect->Width(),
+                  screenRect->Height());
+  }
+  return rect.forget();
+}
+
+void XULPopupElement::SetConstraintRect(dom::DOMRectReadOnly& aRect) {
+  nsMenuPopupFrame* menuPopupFrame =
+      do_QueryFrame(GetPrimaryFrame(FlushType::Frames));
+  if (menuPopupFrame) {
+    menuPopupFrame->SetOverrideConstraintRect(CSSIntRect::Truncate(
+        aRect.Left(), aRect.Top(), aRect.Width(), aRect.Height()));
+  }
+}
+
+bool XULPopupElement::IsWaylandDragSource() const {
+#ifdef MOZ_WAYLAND
+  nsMenuPopupFrame* f = do_QueryFrame(GetPrimaryFrame());
+  return f && f->IsDragSource();
+#else
+  return false;
+#endif
+}
+
+bool XULPopupElement::IsWaylandPopup() const {
+#ifdef MOZ_WAYLAND
+  return widget::GdkIsWaylandDisplay() || widget::IsXWaylandProtocol();
+#else
+  return false;
+#endif
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULPopupElement.h b/dom/xul/XULPopupElement.h
new file mode 100644
index 0000000000..93095a7d51
--- /dev/null
+++ b/dom/xul/XULPopupElement.h
@@ -0,0 +1,116 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULPopupElement_h__
+#define XULPopupElement_h__
+
+#include "XULMenuParentElement.h"
+#include "mozilla/Attributes.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsINode.h"
+#include "nsWrapperCache.h"
+#include "nsString.h"
+#include "nsXULElement.h"
+
+class nsMenuPopupFrame;
+struct JSContext;
+
+namespace mozilla {
+class ErrorResult;
+
+namespace dom {
+
+class DOMRect;
+class Element;
+class Event;
+class StringOrOpenPopupOptions;
+struct ActivateMenuItemOptions;
+
+nsXULElement* NS_NewXULPopupElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+class XULPopupElement : public XULMenuParentElement {
+ private:
+  nsMenuPopupFrame* GetFrame(FlushType);
+
+ public:
+  explicit XULPopupElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+      : XULMenuParentElement(std::move(aNodeInfo)) {}
+
+  void GetLabel(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::label, aValue);
+  }
+  void SetLabel(const nsAString& aValue, ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::label, aValue, rv);
+  }
+
+  bool IsMenu() const { return IsXULElement(nsGkAtoms::menupopup); }
+
+  void GetPosition(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::position, aValue);
+  }
+  void SetPosition(const nsAString& aValue, ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::position, aValue, rv);
+  }
+
+  void OpenPopup(Element* aAnchorElement,
+                 const StringOrOpenPopupOptions& aOptions, int32_t aXPos,
+                 int32_t aYPos, bool aIsContextMenu, bool aAttributesOverride,
+                 Event* aTriggerEvent);
+
+  void OpenPopupAtScreen(int32_t aXPos, int32_t aYPos, bool aIsContextMenu,
+                         Event* aTriggerEvent);
+
+  void OpenPopupAtScreenRect(const nsAString& aPosition, int32_t aXPos,
+                             int32_t aYPos, int32_t aWidth, int32_t aHeight,
+                             bool aIsContextMenu, bool aAttributesOverride,
+                             Event* aTriggerEvent);
+
+  void HidePopup(bool aCancel);
+
+  MOZ_CAN_RUN_SCRIPT void ActivateItem(Element& aItemElement,
+                                       const ActivateMenuItemOptions& aOptions,
+                                       ErrorResult& aRv);
+
+  void GetState(nsString& aState);
+
+  MOZ_CAN_RUN_SCRIPT void PopupOpened(bool aSelectFirstItem);
+  MOZ_CAN_RUN_SCRIPT void PopupClosed(bool aDeselectMenu);
+
+  nsINode* GetTriggerNode() const;
+
+  Element* GetAnchorNode() const;
+
+  already_AddRefed<DOMRect> GetOuterScreenRect();
+
+  void MoveTo(int32_t aLeft, int32_t aTop);
+
+  void MoveToAnchor(Element* aAnchorElement, const nsAString& aPosition,
+                    int32_t aXPos, int32_t aYPos, bool aAttributesOverride);
+
+  void SizeTo(int32_t aWidth, int32_t aHeight);
+
+  void SetConstraintRect(DOMRectReadOnly& aRect);
+
+  bool IsWaylandDragSource() const;
+  bool IsWaylandPopup() const;
+
+  NS_IMPL_FROMNODE_HELPER(XULPopupElement,
+                          IsAnyOfXULElements(nsGkAtoms::menupopup,
+                                             nsGkAtoms::panel,
+                                             nsGkAtoms::tooltip));
+
+ protected:
+  virtual ~XULPopupElement() = default;
+
+  JSObject* WrapNode(JSContext* aCx,
+                     JS::Handle<JSObject*> aGivenProto) override;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // XULPopupElement_h
diff --git a/dom/xul/XULResizerElement.cpp b/dom/xul/XULResizerElement.cpp
new file mode 100644
index 0000000000..a0dbb485d5
--- /dev/null
+++ b/dom/xul/XULResizerElement.cpp
@@ -0,0 +1,354 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/dom/XULResizerElement.h"
+#include "mozilla/dom/XULResizerElementBinding.h"
+
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/MouseEvents.h"
+#include "nsContentUtils.h"
+#include "nsICSSDeclaration.h"
+#include "nsIFrame.h"
+#include "nsLayoutUtils.h"
+#include "nsPresContext.h"
+#include "nsStyledElement.h"
+
+namespace mozilla::dom {
+
+nsXULElement* NS_NewXULResizerElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo) {
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
+  auto* nim = nodeInfo->NodeInfoManager();
+  return new (nim) XULResizerElement(nodeInfo.forget());
+}
+
+static bool GetEventPoint(const WidgetGUIEvent* aEvent,
+                          LayoutDeviceIntPoint& aPoint) {
+  NS_ENSURE_TRUE(aEvent, false);
+
+  const WidgetTouchEvent* touchEvent = aEvent->AsTouchEvent();
+  if (touchEvent) {
+    // return false if there is more than one touch on the page, or if
+    // we can't find a touch point
+    if (touchEvent->mTouches.Length() != 1) {
+      return false;
+    }
+
+    const dom::Touch* touch = touchEvent->mTouches.SafeElementAt(0);
+    if (!touch) {
+      return false;
+    }
+    aPoint = touch->mRefPoint;
+  } else {
+    aPoint = aEvent->mRefPoint;
+  }
+  return true;
+}
+
+JSObject* XULResizerElement::WrapNode(JSContext* aCx,
+                                      JS::Handle<JSObject*> aGivenProto) {
+  return XULResizerElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+XULResizerElement::Direction XULResizerElement::GetDirection() {
+  static const mozilla::dom::Element::AttrValuesArray strings[] = {
+      // clang-format off
+     nsGkAtoms::topleft,    nsGkAtoms::top,    nsGkAtoms::topright,
+     nsGkAtoms::left,                          nsGkAtoms::right,
+     nsGkAtoms::bottomleft, nsGkAtoms::bottom, nsGkAtoms::bottomright,
+     nsGkAtoms::bottomstart,                   nsGkAtoms::bottomend,
+     nullptr
+      // clang-format on
+  };
+
+  static const Direction directions[] = {
+      // clang-format off
+     {-1, -1}, {0, -1}, {1, -1},
+     {-1,  0},          {1,  0},
+     {-1,  1}, {0,  1}, {1,  1},
+     {-1,  1},          {1,  1}
+      // clang-format on
+  };
+
+  const auto* frame = GetPrimaryFrame();
+  if (!frame) {
+    return directions[0];  // default: topleft
+  }
+
+  int32_t index =
+      FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::dir, strings, eCaseMatters);
+  if (index < 0) {
+    return directions[0];  // default: topleft
+  }
+
+  if (index >= 8) {
+    // Directions 8 and higher are RTL-aware directions and should reverse the
+    // horizontal component if RTL.
+    auto wm = frame->GetWritingMode();
+    if (wm.IsPhysicalRTL()) {
+      Direction direction = directions[index];
+      direction.mHorizontal *= -1;
+      return direction;
+    }
+  }
+
+  return directions[index];
+}
+
+nsresult XULResizerElement::PostHandleEvent(EventChainPostVisitor& aVisitor) {
+  if (aVisitor.mEventStatus != nsEventStatus_eConsumeNoDefault) {
+    PostHandleEventInternal(aVisitor);
+  }
+  return nsXULElement::PostHandleEvent(aVisitor);
+}
+
+Maybe<nsSize> XULResizerElement::GetCurrentSize() const {
+  nsIContent* contentToResize = GetContentToResize();
+  if (!contentToResize) {
+    return Nothing();
+  }
+  nsIFrame* frame = contentToResize->GetPrimaryFrame();
+  if (!frame) {
+    return Nothing();
+  }
+  return Some(frame->StylePosition()->mBoxSizing == StyleBoxSizing::Content
+                  ? frame->GetContentRect().Size()
+                  : frame->GetRect().Size());
+}
+
+void XULResizerElement::PostHandleEventInternal(
+    EventChainPostVisitor& aVisitor) {
+  bool doDefault = true;
+  const WidgetEvent& event = *aVisitor.mEvent;
+  switch (event.mMessage) {
+    case eTouchStart:
+    case eMouseDown: {
+      if (event.mClass == eTouchEventClass ||
+          (event.mClass == eMouseEventClass &&
+           event.AsMouseEvent()->mButton == MouseButton::ePrimary)) {
+        auto size = GetCurrentSize();
+        if (!size) {
+          break;  // don't do anything if there's nothing to resize
+        }
+        // cache the content rectangle for the frame to resize
+        mMouseDownSize = *size;
+
+        // remember current mouse coordinates
+        auto* guiEvent = event.AsGUIEvent();
+        if (!GetEventPoint(guiEvent, mMouseDownPoint)) {
+          break;
+        }
+        mTrackingMouseMove = true;
+        PresShell::SetCapturingContent(this, CaptureFlags::IgnoreAllowedState);
+        doDefault = false;
+      }
+    } break;
+
+    case eTouchMove:
+    case eMouseMove: {
+      if (mTrackingMouseMove) {
+        nsCOMPtr<nsIContent> contentToResize = GetContentToResize();
+        if (!contentToResize) {
+          break;  // don't do anything if there's nothing to resize
+        }
+        nsIFrame* frame = contentToResize->GetPrimaryFrame();
+        if (!frame) {
+          break;
+        }
+
+        // both MouseMove and direction are negative when pointing to the
+        // top and left, and positive when pointing to the bottom and right
+
+        // retrieve the offset of the mousemove event relative to the mousedown.
+        // The difference is how much the resize needs to be
+        LayoutDeviceIntPoint refPoint;
+        auto* guiEvent = event.AsGUIEvent();
+        if (!GetEventPoint(guiEvent, refPoint)) {
+          break;
+        }
+
+        const nsPoint oldPos = nsLayoutUtils::GetEventCoordinatesRelativeTo(
+            guiEvent->mWidget, mMouseDownPoint, RelativeTo{frame});
+        const nsPoint newPos = nsLayoutUtils::GetEventCoordinatesRelativeTo(
+            guiEvent->mWidget, refPoint, RelativeTo{frame});
+
+        nsPoint mouseMove(newPos - oldPos);
+
+        // Determine which direction to resize by checking the dir attribute.
+        // For windows and menus, ensure that it can be resized in that
+        // direction.
+        Direction direction = GetDirection();
+
+        const CSSIntSize newSize = [&] {
+          nsSize newAuSize = mMouseDownSize;
+          // Check if there are any size constraints on this window.
+          newAuSize.width += direction.mHorizontal * mouseMove.x;
+          newAuSize.height += direction.mVertical * mouseMove.y;
+          if (newAuSize.width < AppUnitsPerCSSPixel() && mouseMove.x != 0) {
+            newAuSize.width = AppUnitsPerCSSPixel();
+          }
+          if (newAuSize.height < AppUnitsPerCSSPixel() && mouseMove.y != 0) {
+            newAuSize.height = AppUnitsPerCSSPixel();
+          }
+
+          // When changing the size in a direction, don't allow the new size to
+          // be less that the resizer's size. This ensures that content isn't
+          // resized too small as to make the resizer invisible.
+          if (auto* resizerFrame = GetPrimaryFrame()) {
+            nsRect resizerRect = resizerFrame->GetRect();
+            if (newAuSize.width < resizerRect.width && mouseMove.x != 0) {
+              newAuSize.width = resizerRect.width;
+            }
+            if (newAuSize.height < resizerRect.height && mouseMove.y != 0) {
+              newAuSize.height = resizerRect.height;
+            }
+          }
+
+          // Convert the rectangle into css pixels.
+          return CSSIntSize::FromAppUnitsRounded(newAuSize);
+        }();
+
+        // Only resize in a given direction if the new size doesn't match the
+        // current size.
+        if (auto currentSize = GetCurrentSize()) {
+          auto newAuSize = CSSIntSize::ToAppUnits(newSize);
+          if (newAuSize.width == currentSize->width) {
+            direction.mHorizontal = 0;
+          }
+          if (newAuSize.height == currentSize->height) {
+            direction.mVertical = 0;
+          }
+        }
+
+        SizeInfo sizeInfo, originalSizeInfo;
+        sizeInfo.width.AppendInt(newSize.width);
+        sizeInfo.height.AppendInt(newSize.height);
+        ResizeContent(contentToResize, direction, sizeInfo, &originalSizeInfo);
+        MaybePersistOriginalSize(contentToResize, originalSizeInfo);
+
+        doDefault = false;
+      }
+    } break;
+
+    case eMouseClick: {
+      auto* mouseEvent = event.AsMouseEvent();
+      if (mouseEvent->IsLeftClickEvent()) {
+        // Execute the oncommand event handler.
+        nsContentUtils::DispatchXULCommand(
+            this, false, nullptr, nullptr, mouseEvent->IsControl(),
+            mouseEvent->IsAlt(), mouseEvent->IsShift(), mouseEvent->IsMeta(),
+            mouseEvent->mInputSource, mouseEvent->mButton);
+      }
+    } break;
+
+    case eTouchEnd:
+    case eMouseUp: {
+      if (event.mClass == eTouchEventClass ||
+          (event.mClass == eMouseEventClass &&
+           event.AsMouseEvent()->mButton == MouseButton::ePrimary)) {
+        mTrackingMouseMove = false;
+        PresShell::ReleaseCapturingContent();
+        doDefault = false;
+      }
+    } break;
+
+    case eMouseDoubleClick: {
+      if (event.AsMouseEvent()->mButton == MouseButton::ePrimary) {
+        if (nsIContent* contentToResize = GetContentToResize()) {
+          RestoreOriginalSize(contentToResize);
+        }
+      }
+    } break;
+
+    default:
+      break;
+  }
+
+  if (!doDefault) {
+    aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+  }
+}
+
+nsIContent* XULResizerElement::GetContentToResize() const {
+  if (!IsInComposedDoc()) {
+    return nullptr;
+  }
+  // Return the parent, but skip over native anonymous content
+  nsIContent* parent = GetParent();
+  return parent ? parent->FindFirstNonChromeOnlyAccessContent() : nullptr;
+}
+
+/* static */
+void XULResizerElement::ResizeContent(nsIContent* aContent,
+                                      const Direction& aDirection,
+                                      const SizeInfo& aSizeInfo,
+                                      SizeInfo* aOriginalSizeInfo) {
+  RefPtr inlineStyleContent = nsStyledElement::FromNode(aContent);
+  if (!inlineStyleContent) {
+    return;
+  }
+  nsCOMPtr<nsICSSDeclaration> decl = inlineStyleContent->Style();
+  if (aOriginalSizeInfo) {
+    decl->GetPropertyValue("width"_ns, aOriginalSizeInfo->width);
+    decl->GetPropertyValue("height"_ns, aOriginalSizeInfo->height);
+  }
+
+  // only set the property if the element could have changed in that
+  // direction
+  if (aDirection.mHorizontal) {
+    nsAutoCString widthstr(aSizeInfo.width);
+    if (!widthstr.IsEmpty() && !StringEndsWith(widthstr, "px"_ns)) {
+      widthstr.AppendLiteral("px");
+    }
+    decl->SetProperty("width"_ns, widthstr, ""_ns, IgnoreErrors());
+  }
+
+  if (aDirection.mVertical) {
+    nsAutoCString heightstr(aSizeInfo.height);
+    if (!heightstr.IsEmpty() && !StringEndsWith(heightstr, "px"_ns)) {
+      heightstr.AppendLiteral("px");
+    }
+    decl->SetProperty("height"_ns, heightstr, ""_ns, IgnoreErrors());
+  }
+}
+
+/* static */
+void XULResizerElement::MaybePersistOriginalSize(nsIContent* aContent,
+                                                 const SizeInfo& aSizeInfo) {
+  nsresult rv;
+  aContent->GetProperty(nsGkAtoms::_moz_original_size, &rv);
+  if (rv != NS_PROPTABLE_PROP_NOT_THERE) {
+    return;
+  }
+
+  UniquePtr<SizeInfo> sizeInfo(new SizeInfo(aSizeInfo));
+  rv = aContent->SetProperty(
+      nsGkAtoms::_moz_original_size, sizeInfo.get(),
+      nsINode::DeleteProperty<XULResizerElement::SizeInfo>);
+  if (NS_SUCCEEDED(rv)) {
+    Unused << sizeInfo.release();
+  }
+}
+
+/* static */
+void XULResizerElement::RestoreOriginalSize(nsIContent* aContent) {
+  nsresult rv;
+  SizeInfo* sizeInfo = static_cast<SizeInfo*>(
+      aContent->GetProperty(nsGkAtoms::_moz_original_size, &rv));
+  if (NS_FAILED(rv)) {
+    return;
+  }
+
+  NS_ASSERTION(sizeInfo, "We set a null sizeInfo!?");
+  Direction direction = {1, 1};
+  ResizeContent(aContent, direction, *sizeInfo, nullptr);
+  aContent->RemoveProperty(nsGkAtoms::_moz_original_size);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULResizerElement.h b/dom/xul/XULResizerElement.h
new file mode 100644
index 0000000000..f7c9e4d75d
--- /dev/null
+++ b/dom/xul/XULResizerElement.h
@@ -0,0 +1,62 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_dom_XULResizerElement_h
+#define mozilla_dom_XULResizerElement_h
+
+#include "nsXULElement.h"
+#include "Units.h"
+
+namespace mozilla::dom {
+
+nsXULElement* NS_NewXULResizerElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+class XULResizerElement final : public nsXULElement {
+ public:
+  explicit XULResizerElement(already_AddRefed<dom::NodeInfo>&& aNodeInfo)
+      : nsXULElement(std::move(aNodeInfo)) {}
+
+  MOZ_CAN_RUN_SCRIPT
+  nsresult PostHandleEvent(EventChainPostVisitor&) override;
+
+ private:
+  virtual ~XULResizerElement() = default;
+  JSObject* WrapNode(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) final;
+
+  MOZ_CAN_RUN_SCRIPT
+  void PostHandleEventInternal(EventChainPostVisitor&);
+
+  struct Direction {
+    int8_t mHorizontal;
+    int8_t mVertical;
+  };
+  Direction GetDirection();
+
+  nsIContent* GetContentToResize() const;
+  // The current size of the content to resize (if available).
+  Maybe<nsSize> GetCurrentSize() const;
+
+  struct SizeInfo {
+    nsCString width, height;
+  };
+  static void SizeInfoDtorFunc(void* aObject, nsAtom* aPropertyName,
+                               void* aPropertyValue, void* aData);
+  static void ResizeContent(nsIContent* aContent, const Direction& aDirection,
+                            const SizeInfo& aSizeInfo,
+                            SizeInfo* aOriginalSizeInfo);
+  static void MaybePersistOriginalSize(nsIContent* aContent,
+                                       const SizeInfo& aSizeInfo);
+  static void RestoreOriginalSize(nsIContent* aContent);
+
+  nsSize mMouseDownSize;
+  LayoutDeviceIntPoint mMouseDownPoint;
+  bool mTrackingMouseMove = false;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // XULResizerElement_h
diff --git a/dom/xul/XULTextElement.cpp b/dom/xul/XULTextElement.cpp
new file mode 100644
index 0000000000..04edfabc09
--- /dev/null
+++ b/dom/xul/XULTextElement.cpp
@@ -0,0 +1,49 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/dom/Element.h"
+#include "mozilla/dom/ToJSValue.h"
+#include "nsCOMPtr.h"
+#include "nsChangeHint.h"
+#include "nsIContent.h"
+#include "nsPresContext.h"
+#include "nsIScrollableFrame.h"
+#include "mozilla/dom/MutationEventBinding.h"
+#include "mozilla/dom/XULTextElement.h"
+#include "mozilla/dom/XULTextElementBinding.h"
+
+namespace mozilla::dom {
+
+nsChangeHint XULTextElement::GetAttributeChangeHint(const nsAtom* aAttribute,
+                                                    int32_t aModType) const {
+  const bool reframe = [&] {
+    if (aAttribute == nsGkAtoms::value) {
+      // If we have an accesskey we need to recompute our -moz-label-content.
+      // Otherwise this is handled by either the attribute text node, or
+      // nsTextBoxFrame for crop="center".
+      return aModType == MutationEvent_Binding::ADDITION ||
+             aModType == MutationEvent_Binding::REMOVAL ||
+             HasAttr(nsGkAtoms::accesskey);
+    }
+    if (aAttribute == nsGkAtoms::crop || aAttribute == nsGkAtoms::accesskey) {
+      // value attr + crop="center" still uses nsTextBoxFrame. accesskey
+      // requires reframing as per the above comment.
+      return HasAttr(nsGkAtoms::value);
+    }
+    return false;
+  }();
+  if (reframe) {
+    return nsChangeHint_ReconstructFrame;
+  }
+  return nsXULElement::GetAttributeChangeHint(aAttribute, aModType);
+}
+
+JSObject* XULTextElement::WrapNode(JSContext* aCx,
+                                   JS::Handle<JSObject*> aGivenProto) {
+  return XULTextElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULTextElement.h b/dom/xul/XULTextElement.h
new file mode 100644
index 0000000000..0c6f086b5e
--- /dev/null
+++ b/dom/xul/XULTextElement.h
@@ -0,0 +1,50 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULTextElement_h__
+#define XULTextElement_h__
+
+#include "nsXULElement.h"
+
+namespace mozilla::dom {
+
+class XULTextElement final : public nsXULElement {
+ public:
+  explicit XULTextElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+      : nsXULElement(std::move(aNodeInfo)) {}
+
+  bool Disabled() { return GetXULBoolAttr(nsGkAtoms::disabled); }
+  MOZ_CAN_RUN_SCRIPT void SetDisabled(bool aValue) {
+    SetXULBoolAttr(nsGkAtoms::disabled, aValue, mozilla::IgnoreErrors());
+  }
+  void GetValue(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::value, aValue);
+  }
+  MOZ_CAN_RUN_SCRIPT void SetValue(const nsAString& aValue) {
+    SetAttr(kNameSpaceID_None, nsGkAtoms::value, aValue, true);
+  }
+  void GetAccessKey(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::accesskey, aValue);
+  }
+  MOZ_CAN_RUN_SCRIPT void SetAccessKey(const nsAString& aValue) {
+    SetAttr(kNameSpaceID_None, nsGkAtoms::accesskey, aValue, true);
+  }
+
+  nsChangeHint GetAttributeChangeHint(const nsAtom* aAttribute,
+                                      int32_t aModType) const override;
+
+  NS_IMPL_FROMNODE_HELPER(XULTextElement,
+                          IsAnyOfXULElements(nsGkAtoms::label,
+                                             nsGkAtoms::description));
+
+ protected:
+  virtual ~XULTextElement() = default;
+  JSObject* WrapNode(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) final;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // XULTextElement_h
diff --git a/dom/xul/XULTooltipElement.cpp b/dom/xul/XULTooltipElement.cpp
new file mode 100644
index 0000000000..cf9e46c1ca
--- /dev/null
+++ b/dom/xul/XULTooltipElement.cpp
@@ -0,0 +1,104 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsCOMPtr.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/XULTooltipElement.h"
+#include "mozilla/dom/NodeInfo.h"
+#include "mozilla/EventDispatcher.h"
+#include "nsContentCreatorFunctions.h"
+#include "nsContentUtils.h"
+#include "nsCTooltipTextProvider.h"
+#include "nsITooltipTextProvider.h"
+#include "nsServiceManagerUtils.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla::dom {
+
+nsXULElement* NS_NewXULTooltipElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo) {
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
+  auto* nim = nodeInfo->NodeInfoManager();
+  RefPtr<XULTooltipElement> tooltip =
+      new (nim) XULTooltipElement(nodeInfo.forget());
+  NS_ENSURE_SUCCESS(tooltip->Init(), nullptr);
+  return tooltip;
+}
+
+nsresult XULTooltipElement::Init() {
+  // Create the default child label node that will contain the text of the
+  // tooltip.
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo;
+  nodeInfo = mNodeInfo->NodeInfoManager()->GetNodeInfo(
+      nsGkAtoms::description, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
+  nsCOMPtr<Element> description;
+  nsresult rv = NS_NewXULElement(getter_AddRefs(description), nodeInfo.forget(),
+                                 dom::NOT_FROM_PARSER);
+  NS_ENSURE_SUCCESS(rv, rv);
+  description->SetAttr(kNameSpaceID_None, nsGkAtoms::_class,
+                       u"tooltip-label"_ns, false);
+  ErrorResult error;
+  AppendChild(*description, error);
+
+  return error.StealNSResult();
+}
+
+void XULTooltipElement::AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
+                                     const nsAttrValue* aValue,
+                                     const nsAttrValue* aOldValue,
+                                     nsIPrincipal* aSubjectPrincipal,
+                                     bool aNotify) {
+  if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::label) {
+    // When the label attribute of this node changes propagate the text down
+    // into child description element.
+    nsCOMPtr<nsIContent> description = GetFirstChild();
+    if (description && description->IsXULElement(nsGkAtoms::description)) {
+      nsAutoString value;
+      if (aValue) {
+        aValue->ToString(value);
+      }
+      nsContentUtils::AddScriptRunner(NS_NewRunnableFunction(
+          "XULTooltipElement::AfterSetAttr", [description, value]() {
+            Element* descriptionElement = description->AsElement();
+            descriptionElement->SetTextContent(value, IgnoreErrors());
+          }));
+    }
+  }
+  return nsXULElement::AfterSetAttr(aNameSpaceID, aName, aValue, aOldValue,
+                                    aSubjectPrincipal, aNotify);
+}
+
+nsresult XULTooltipElement::PostHandleEvent(EventChainPostVisitor& aVisitor) {
+  if (aVisitor.mEvent->mMessage == eXULPopupShowing &&
+      aVisitor.mEvent->IsTrusted() && !aVisitor.mEvent->DefaultPrevented() &&
+      AttrValueIs(kNameSpaceID_None, nsGkAtoms::page, nsGkAtoms::_true,
+                  eCaseMatters) &&
+      !AttrValueIs(kNameSpaceID_None, nsGkAtoms::titletip, nsGkAtoms::_true,
+                   eCaseMatters)) {
+    // When the tooltip node has the "page" attribute set to "true" the
+    // tooltip text provider is used to find the tooltip text from page where
+    // mouse is hovering over.
+    nsCOMPtr<nsITooltipTextProvider> textProvider =
+        do_GetService(NS_DEFAULTTOOLTIPTEXTPROVIDER_CONTRACTID);
+    nsString text;
+    nsString direction;
+    bool shouldChange = false;
+    if (textProvider) {
+      textProvider->GetNodeText(GetTriggerNode(), getter_Copies(text),
+                                getter_Copies(direction), &shouldChange);
+    }
+    if (shouldChange) {
+      SetAttr(kNameSpaceID_None, nsGkAtoms::label, text, true);
+      SetAttr(kNameSpaceID_None, nsGkAtoms::direction, direction, true);
+    } else {
+      aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
+      aVisitor.mEvent->PreventDefault();
+    }
+  }
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULTooltipElement.h b/dom/xul/XULTooltipElement.h
new file mode 100644
index 0000000000..1f39ad7f67
--- /dev/null
+++ b/dom/xul/XULTooltipElement.h
@@ -0,0 +1,35 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULTooltipElement_h__
+#define XULTooltipElement_h__
+
+#include "XULPopupElement.h"
+
+namespace mozilla::dom {
+
+nsXULElement* NS_NewXULTooltipElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+class XULTooltipElement final : public XULPopupElement {
+ public:
+  explicit XULTooltipElement(
+      already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+      : XULPopupElement(std::move(aNodeInfo)) {}
+  nsresult Init();
+
+  void AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
+                    const nsAttrValue* aValue, const nsAttrValue* aOldValue,
+                    nsIPrincipal* aSubjectPrincipal, bool aNotify) override;
+  nsresult PostHandleEvent(EventChainPostVisitor& aVisitor) override;
+
+ protected:
+  virtual ~XULTooltipElement() = default;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // XULPopupElement_h
diff --git a/dom/xul/XULTreeElement.cpp b/dom/xul/XULTreeElement.cpp
new file mode 100644
index 0000000000..e190b22556
--- /dev/null
+++ b/dom/xul/XULTreeElement.cpp
@@ -0,0 +1,411 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsCOMPtr.h"
+#include "nsTreeContentView.h"
+#include "nsITreeSelection.h"
+#include "ChildIterator.h"
+#include "nsError.h"
+#include "nsTreeBodyFrame.h"
+#include "mozilla/dom/DOMRect.h"
+#include "mozilla/dom/BindingUtils.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/ToJSValue.h"
+#include "mozilla/dom/XULTreeElement.h"
+#include "mozilla/dom/XULTreeElementBinding.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(XULTreeElement, nsXULElement)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(XULTreeElement, nsXULElement, mView)
+
+JSObject* XULTreeElement::WrapNode(JSContext* aCx,
+                                   JS::Handle<JSObject*> aGivenProto) {
+  return XULTreeElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+void XULTreeElement::UnbindFromTree(bool aNullParent) {
+  // Drop the view's ref to us.
+  if (mView) {
+    nsCOMPtr<nsITreeSelection> sel;
+    mView->GetSelection(getter_AddRefs(sel));
+    if (sel) {
+      sel->SetTree(nullptr);
+    }
+    mView->SetTree(nullptr);  // Break the circular ref between the view and us.
+  }
+  mView = nullptr;
+
+  nsXULElement::UnbindFromTree(aNullParent);
+}
+
+void XULTreeElement::DestroyContent() {
+  // Drop the view's ref to us.
+  if (mView) {
+    nsCOMPtr<nsITreeSelection> sel;
+    mView->GetSelection(getter_AddRefs(sel));
+    if (sel) {
+      sel->SetTree(nullptr);
+    }
+    mView->SetTree(nullptr);  // Break the circular ref between the view and us.
+  }
+  mView = nullptr;
+
+  nsXULElement::DestroyContent();
+}
+
+static nsIContent* FindBodyElement(nsIContent* aParent) {
+  mozilla::dom::FlattenedChildIterator iter(aParent);
+  for (nsIContent* content = iter.GetNextChild(); content;
+       content = iter.GetNextChild()) {
+    mozilla::dom::NodeInfo* ni = content->NodeInfo();
+    if (ni->Equals(nsGkAtoms::treechildren, kNameSpaceID_XUL)) {
+      return content;
+    } else if (ni->Equals(nsGkAtoms::tree, kNameSpaceID_XUL)) {
+      // There are nesting tree elements. Only the innermost should
+      // find the treechilren.
+      return nullptr;
+    } else if (content->IsElement() &&
+               !ni->Equals(nsGkAtoms::_template, kNameSpaceID_XUL)) {
+      nsIContent* result = FindBodyElement(content);
+      if (result) return result;
+    }
+  }
+
+  return nullptr;
+}
+
+nsTreeBodyFrame* XULTreeElement::GetTreeBodyFrame(FlushType aFlushType) {
+  MOZ_ASSERT(aFlushType == FlushType::Frames ||
+             aFlushType == FlushType::Layout || aFlushType == FlushType::None);
+  nsCOMPtr<nsIContent> kungFuDeathGrip = this;  // keep a reference
+
+  // Make sure our frames are up to date, and layout as needed.  We
+  // have to do this before checking for our cached mTreeBody, since
+  // it might go away on style flush, and in any case if aFlushLayout
+  // is true we need to make sure to flush no matter what.
+  if (aFlushType != FlushType::None) {
+    if (RefPtr<Document> doc = GetComposedDoc()) {
+      doc->FlushPendingNotifications(aFlushType);
+    }
+  }
+
+  if (mTreeBody) {
+    // Have one cached already.
+    return mTreeBody;
+  }
+
+  if (nsCOMPtr<nsIContent> tree = FindBodyElement(this)) {
+    mTreeBody = do_QueryFrame(tree->GetPrimaryFrame());
+  }
+
+  return mTreeBody;
+}
+
+already_AddRefed<nsITreeView> XULTreeElement::GetView(FlushType aFlushType) {
+  if (!mTreeBody) {
+    if (!GetTreeBodyFrame(aFlushType)) {
+      return nullptr;
+    }
+
+    if (mView) {
+      nsCOMPtr<nsITreeView> view;
+      // Our new frame needs to initialise itself
+      mTreeBody->GetView(getter_AddRefs(view));
+      return view.forget();
+    }
+  }
+  if (!mView) {
+    // No tree builder, create a tree content view.
+    if (NS_FAILED(NS_NewTreeContentView(getter_AddRefs(mView)))) {
+      return nullptr;
+    }
+
+    // Initialise the frame and view
+    mTreeBody->SetView(mView);
+  }
+
+  return do_AddRef(mView);
+}
+
+void XULTreeElement::SetView(nsITreeView* aView, CallerType aCallerType,
+                             ErrorResult& aRv) {
+  if (aCallerType != CallerType::System) {
+    // Don't trust views coming from random places.
+    aRv.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  mView = aView;
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->SetView(aView);
+  }
+}
+
+bool XULTreeElement::Focused() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->GetFocused();
+  }
+  return false;
+}
+
+void XULTreeElement::SetFocused(bool aFocused) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->SetFocused(aFocused);
+  }
+}
+
+already_AddRefed<Element> XULTreeElement::GetTreeBody() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    nsCOMPtr<Element> element;
+    body->GetTreeBody(getter_AddRefs(element));
+    return element.forget();
+  }
+
+  return nullptr;
+}
+
+already_AddRefed<nsTreeColumns> XULTreeElement::GetColumns(
+    FlushType aFlushType) {
+  if (nsTreeBodyFrame* body = GetTreeBodyFrame(aFlushType)) {
+    return body->Columns();
+  }
+  return nullptr;
+}
+
+int32_t XULTreeElement::RowHeight() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->RowHeight();
+  }
+  return 0;
+}
+
+int32_t XULTreeElement::RowWidth() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->RowWidth();
+  }
+  return 0;
+}
+
+int32_t XULTreeElement::GetFirstVisibleRow() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->FirstVisibleRow();
+  }
+  return 0;
+}
+
+int32_t XULTreeElement::GetLastVisibleRow() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->LastVisibleRow();
+  }
+  return 0;
+}
+
+int32_t XULTreeElement::HorizontalPosition() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->GetHorizontalPosition();
+  }
+  return 0;
+}
+
+int32_t XULTreeElement::GetPageLength() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    return body->PageLength();
+  }
+  return 0;
+}
+
+void XULTreeElement::EnsureRowIsVisible(int32_t aRow) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->EnsureRowIsVisible(aRow);
+  }
+}
+
+void XULTreeElement::EnsureCellIsVisible(int32_t aRow, nsTreeColumn* aCol,
+                                         ErrorResult& aRv) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    nsresult rv = body->EnsureCellIsVisible(aRow, aCol);
+    if (NS_FAILED(rv)) {
+      aRv.Throw(rv);
+    }
+  }
+}
+
+void XULTreeElement::ScrollToRow(int32_t aRow) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame(FlushType::Layout);
+  if (!body) {
+    return;
+  }
+
+  body->ScrollToRow(aRow);
+}
+
+void XULTreeElement::ScrollByLines(int32_t aNumLines) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (!body) {
+    return;
+  }
+  body->ScrollByLines(aNumLines);
+}
+
+void XULTreeElement::ScrollByPages(int32_t aNumPages) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->ScrollByPages(aNumPages);
+  }
+}
+
+void XULTreeElement::Invalidate() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->Invalidate();
+  }
+}
+
+void XULTreeElement::InvalidateColumn(nsTreeColumn* aCol) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->InvalidateColumn(aCol);
+  }
+}
+
+void XULTreeElement::InvalidateRow(int32_t aIndex) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->InvalidateRow(aIndex);
+  }
+}
+
+void XULTreeElement::InvalidateCell(int32_t aRow, nsTreeColumn* aCol) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->InvalidateCell(aRow, aCol);
+  }
+}
+
+void XULTreeElement::InvalidateRange(int32_t aStart, int32_t aEnd) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->InvalidateRange(aStart, aEnd);
+  }
+}
+
+int32_t XULTreeElement::GetRowAt(int32_t x, int32_t y) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (!body) {
+    return 0;
+  }
+  return body->GetRowAt(x, y);
+}
+
+void XULTreeElement::GetCellAt(int32_t aX, int32_t aY, TreeCellInfo& aRetVal,
+                               ErrorResult& aRv) {
+  aRetVal.mRow = 0;
+  aRetVal.mCol = nullptr;
+
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    nsAutoCString element;
+    body->GetCellAt(aX, aY, &aRetVal.mRow, getter_AddRefs(aRetVal.mCol),
+                    element);
+    CopyUTF8toUTF16(element, aRetVal.mChildElt);
+  }
+}
+
+nsIntRect XULTreeElement::GetCoordsForCellItem(int32_t aRow, nsTreeColumn* aCol,
+                                               const nsAString& aElement,
+                                               nsresult& rv) {
+  rv = NS_OK;
+  nsIntRect rect;
+
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  NS_ConvertUTF16toUTF8 element(aElement);
+  if (body) {
+    rv = body->GetCoordsForCellItem(aRow, aCol, element, &rect.x, &rect.y,
+                                    &rect.width, &rect.height);
+  }
+
+  return rect;
+}
+
+already_AddRefed<DOMRect> XULTreeElement::GetCoordsForCellItem(
+    int32_t aRow, nsTreeColumn& aCol, const nsAString& aElement,
+    ErrorResult& aRv) {
+  nsresult rv;
+  nsIntRect rect = GetCoordsForCellItem(aRow, &aCol, aElement, rv);
+  aRv = rv;
+
+  RefPtr<DOMRect> domRect = new DOMRect(ToSupports(OwnerDoc()), rect.x, rect.y,
+                                        rect.width, rect.height);
+  return domRect.forget();
+}
+
+bool XULTreeElement::IsCellCropped(int32_t aRow, nsTreeColumn* aCol,
+                                   ErrorResult& aRv) {
+  bool cropped = false;
+
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    aRv = body->IsCellCropped(aRow, aCol, &cropped);
+  }
+
+  return cropped;
+}
+
+void XULTreeElement::RowCountChanged(int32_t aIndex, int32_t aDelta) {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->RowCountChanged(aIndex, aDelta);
+  }
+}
+
+void XULTreeElement::BeginUpdateBatch() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->BeginUpdateBatch();
+  }
+}
+
+void XULTreeElement::EndUpdateBatch() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->EndUpdateBatch();
+  }
+}
+
+void XULTreeElement::ClearStyleAndImageCaches() {
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->ClearStyleAndImageCaches();
+  }
+}
+
+void XULTreeElement::RemoveImageCacheEntry(int32_t aRowIndex,
+                                           nsTreeColumn& aCol,
+                                           ErrorResult& aRv) {
+  if (NS_WARN_IF(aRowIndex < 0)) {
+    aRv.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+  nsTreeBodyFrame* body = GetTreeBodyFrame();
+  if (body) {
+    body->RemoveImageCacheEntry(aRowIndex, &aCol);
+  }
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/xul/XULTreeElement.h b/dom/xul/XULTreeElement.h
new file mode 100644
index 0000000000..381ae88f41
--- /dev/null
+++ b/dom/xul/XULTreeElement.h
@@ -0,0 +1,129 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef XULTreeElement_h__
+#define XULTreeElement_h__
+
+#include "mozilla/Attributes.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "nsString.h"
+#include "nsXULElement.h"
+#include "nsITreeView.h"
+
+class nsTreeBodyFrame;
+class nsTreeColumn;
+class nsTreeColumns;
+
+namespace mozilla {
+class ErrorResult;
+
+namespace dom {
+
+struct TreeCellInfo;
+class DOMRect;
+enum class CallerType : uint32_t;
+
+class XULTreeElement final : public nsXULElement {
+ public:
+  explicit XULTreeElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+      : nsXULElement(std::move(aNodeInfo)),
+        mCachedFirstVisibleRow(0),
+        mTreeBody(nullptr) {}
+
+  NS_IMPL_FROMNODE_WITH_TAG(XULTreeElement, kNameSpaceID_XUL, tree)
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(XULTreeElement, nsXULElement)
+
+  nsTreeBodyFrame* GetTreeBodyFrame(FlushType = FlushType::Frames);
+  nsTreeBodyFrame* GetCachedTreeBodyFrame() { return mTreeBody; }
+
+  already_AddRefed<nsTreeColumns> GetColumns(FlushType = FlushType::Frames);
+
+  already_AddRefed<nsITreeView> GetView(CallerType /* unused */) {
+    return GetView();
+  }
+  already_AddRefed<nsITreeView> GetView(FlushType = FlushType::Frames);
+
+  void SetView(nsITreeView* arg, CallerType aCallerType, ErrorResult& aRv);
+
+  bool Focused();
+
+  already_AddRefed<Element> GetTreeBody();
+
+  int32_t RowHeight();
+
+  int32_t RowWidth();
+
+  int32_t HorizontalPosition();
+
+  void EnsureCellIsVisible(int32_t row, nsTreeColumn* col, ErrorResult& aRv);
+
+  void ScrollToRow(int32_t aRow);
+
+  void ScrollByLines(int32_t aNumLines);
+
+  void ScrollByPages(int32_t aNumPages);
+
+  int32_t GetFirstVisibleRow();
+
+  int32_t GetLastVisibleRow();
+
+  int32_t GetPageLength();
+
+  int32_t GetRowAt(int32_t x, int32_t y);
+
+  void GetCellAt(int32_t x, int32_t y, TreeCellInfo& aRetVal, ErrorResult& aRv);
+
+  nsIntRect GetCoordsForCellItem(int32_t aRow, nsTreeColumn* aCol,
+                                 const nsAString& aElement, nsresult& rv);
+  already_AddRefed<DOMRect> GetCoordsForCellItem(int32_t row, nsTreeColumn& col,
+                                                 const nsAString& element,
+                                                 ErrorResult& aRv);
+
+  bool IsCellCropped(int32_t row, nsTreeColumn* col, ErrorResult& aRv);
+
+  void RemoveImageCacheEntry(int32_t row, nsTreeColumn& col, ErrorResult& aRv);
+
+  void SetFocused(bool aFocused);
+  void EnsureRowIsVisible(int32_t index);
+  void Invalidate(void);
+  void InvalidateColumn(nsTreeColumn* col);
+  void InvalidateRow(int32_t index);
+  void InvalidateCell(int32_t row, nsTreeColumn* col);
+  void InvalidateRange(int32_t startIndex, int32_t endIndex);
+  void RowCountChanged(int32_t index, int32_t count);
+  void BeginUpdateBatch(void);
+  void EndUpdateBatch(void);
+  void ClearStyleAndImageCaches(void);
+
+  virtual void UnbindFromTree(bool aNullParent) override;
+  virtual void DestroyContent() override;
+
+  void BodyDestroyed(int32_t aFirstVisibleRow) {
+    mTreeBody = nullptr;
+    mCachedFirstVisibleRow = aFirstVisibleRow;
+  }
+
+  int32_t GetCachedTopVisibleRow() { return mCachedFirstVisibleRow; }
+
+ protected:
+  int32_t mCachedFirstVisibleRow;
+
+  nsTreeBodyFrame* mTreeBody;
+  nsCOMPtr<nsITreeView> mView;
+
+  virtual ~XULTreeElement() = default;
+
+  JSObject* WrapNode(JSContext* aCx,
+                     JS::Handle<JSObject*> aGivenProto) override;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif
diff --git a/dom/xul/crashtests/107518-1.xml b/dom/xul/crashtests/107518-1.xml
new file mode 100644
index 0000000000..58e652624c
--- /dev/null
+++ b/dom/xul/crashtests/107518-1.xml
@@ -0,0 +1,52 @@
+<?xml version="1.0"?>

+<?xml-stylesheet href="chrome://global/skin/" type="text/css"?> 

+<!DOCTYPE window>

+

+<window

+   id         = "xulnote-main-window"

+   xmlns      = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"

+   xmlns:html = "http://www.w3.org/1999/xhtml" 

+   onload     = "init();"

+>

+  <scrollbox>

+   <vbox style="background-color:white;">

+      <label value="hahaha  0"/>

+      <label value="hahaha  1"/>

+      <label value="hahaha  2"/>

+      <label value="hahaha  3"/>

+      <label value="hahaha  4"/>

+      <label value="hahaha  5"/>

+      <label value="hahaha  6"/>

+      <label value="hahaha  7"/>

+      <label value="hahaha  8"/>

+      <label value="hahaha  9"/>

+      <label value="hahaha 10"/>

+      <label value="hahaha 11"/>

+      <label value="hahaha 12"/>

+      <label value="hahaha 13"/>

+      <label value="hahaha 14"/>

+      <label value="hahaha 15"/>

+      <label value="hahaha 16"/>

+      <label value="hahaha 17"/>

+      <label value="hahaha 18"/>

+      <label value="hahaha 19"/>

+   </vbox>

+<scrollbar

+    id="identifier"

+    align="horizontal"

+    curpos="20"

+    maxpos="100"

+    increment="1"

+    pageincrement="10"/>

+

+  </scrollbox>

+

+ <script type="application/x-javascript">

+  <![CDATA[

+   function init()

+   {

+   }

+ ]]>

+ </script>

+

+</window>

diff --git a/dom/xul/crashtests/253479-1.xhtml b/dom/xul/crashtests/253479-1.xhtml
new file mode 100644
index 0000000000..5860d2200d
--- /dev/null
+++ b/dom/xul/crashtests/253479-1.xhtml
@@ -0,0 +1,6 @@
+<?xml version="1.0"?>

+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"

+        xmlns:html="http://www.w3.org/1999/xhtml"

+>

+<html:div style="position:fixed;"/>

+</window>
\ No newline at end of file
diff --git a/dom/xul/crashtests/253479-2.xhtml b/dom/xul/crashtests/253479-2.xhtml
new file mode 100644
index 0000000000..43a8c17b84
--- /dev/null
+++ b/dom/xul/crashtests/253479-2.xhtml
@@ -0,0 +1,4 @@
+<?xml version="1.0"?>

+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

+  <x style="position:fixed; display: block;"/>

+</window>

diff --git a/dom/xul/crashtests/326204-1.xhtml b/dom/xul/crashtests/326204-1.xhtml
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/dom/xul/crashtests/326204-1.xhtml
diff --git a/dom/xul/crashtests/326644-1-inner.xhtml b/dom/xul/crashtests/326644-1-inner.xhtml
new file mode 100644
index 0000000000..63ed9b7212
--- /dev/null
+++ b/dom/xul/crashtests/326644-1-inner.xhtml
@@ -0,0 +1,34 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:html="http://www.w3.org/1999/xhtml"

+ title="Testcase bug 326644 - Crash when changing enumerated properties of objects in xul">

+

+<html:script><![CDATA[

+var timers=0;

+function doe(aObj, aNested, aCurrentTimer){

+var temp =0;

+for (var i in aObj) {

+try {

+if (typeof aObj[i] == 'object') {

+  if (aNested >= 19 || aObj[i] == window.location)

+    continue;

+  setTimeout(doe,500, aObj[i], ++aNested, timers);

+  timers++;

+}

+}

+catch(e){}

+try {

+ //if (temp == 68 && aNested == 21 && aCurrentTimer >= 116) {

+ // alert(i + '-'+ aObj[i]);

+ // return;

+ // }

+ aObj[i]= i;

+ temp+=1;

+}

+catch (e) {

+

+}

+}

+}

+var s=document.getElementsByTagName('window')[0];

+setTimeout(doe,100, s, 0);

+]]></html:script>

+</window>

diff --git a/dom/xul/crashtests/326644-1.html b/dom/xul/crashtests/326644-1.html
new file mode 100644
index 0000000000..6f1d88a5eb
--- /dev/null
+++ b/dom/xul/crashtests/326644-1.html
@@ -0,0 +1,9 @@
+<html class="reftest-wait">
+<head>
+<script>
+setTimeout('document.documentElement.className = ""', 1000);
+</script>
+<body>
+<iframe src="326644-1-inner.xhtml"></iframe>
+</body>
+</html>
diff --git a/dom/xul/crashtests/326875-1.xhtml b/dom/xul/crashtests/326875-1.xhtml
new file mode 100644
index 0000000000..9dca9a0d53
--- /dev/null
+++ b/dom/xul/crashtests/326875-1.xhtml
@@ -0,0 +1,27 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+
+
+<script>
+
+
+function init() { 
+
+  var m = document.getElementById("m"); 
+  m.parentNode.removeChild(m);
+  m.controllers;
+};
+
+
+window.addEventListener("load", init, false);
+
+</script>
+
+
+
+<hbox id="m" />
+    
+</window>
diff --git a/dom/xul/crashtests/329982-1.xhtml b/dom/xul/crashtests/329982-1.xhtml
new file mode 100644
index 0000000000..43f374a57e
--- /dev/null
+++ b/dom/xul/crashtests/329982-1.xhtml
@@ -0,0 +1,42 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+
+<script>
+
+function init()
+{
+  var A = document.getElementById("z");
+  var B = A.nextSibling;
+  var C = B.nextSibling;
+  var P = A.parentNode;
+  
+  document.addEventListener("DOMNodeRemoved", fizzy, false);
+  P.removeChild(B);
+  document.removeEventListener("DOMNodeRemoved", fizzy, false);
+  
+  function fizzy()
+  {
+    document.removeEventListener("DOMNodeRemoved", fizzy, false); // avoid recursion
+    P.removeChild(A);
+  }
+    
+  document.documentElement.appendChild(C);
+}
+
+
+window.addEventListener("load", init, false);
+
+</script>
+
+</head>
+
+<body>
+
+<hbox xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+<menupopup y="x">
  <menuitem id="z"/>
  <menuitem/>
  <menuitem/>
</menupopup>

+
+</hbox>
+
+</body>
+</html>
\ No newline at end of file
diff --git a/dom/xul/crashtests/336096-1.xhtml b/dom/xul/crashtests/336096-1.xhtml
new file mode 100644
index 0000000000..e15691d88d
--- /dev/null
+++ b/dom/xul/crashtests/336096-1.xhtml
@@ -0,0 +1,41 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+<script>
+<![CDATA[
+
+function init()
+{
+    var targetWindow = window.frames[0];
+    var targetDocument = targetWindow.document;
+    
+    targetDocument.body.appendChild(document.getElementById('rootish'));
+    targetDocument.designMode = 'on';
+
+    var r = targetDocument.createRange(); 
+    r.setStart(targetDocument.getElementById("start"), 0); 
+    r.setEnd  (targetDocument.getElementById("end"), 0); 
+    targetWindow.getSelection().addRange(r);
+
+    targetDocument.execCommand('bold', false, null);
+}
+
+]]>
+</script>
+</head>
+
+<body onload="setTimeout(init, 200);">
+
+<iframe src="data:text/html," style="width: 95%; height: 500px;"></iframe>
+
+<div id="rootish">
+  <div id="start"></div>
+  <hbox xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+    <hbox/>
+    <vbox id="end">
+      <hbox/>
+    </vbox>
+  </hbox>
+</div>
+
+</body>
+</html>
diff --git a/dom/xul/crashtests/344215-1.xhtml b/dom/xul/crashtests/344215-1.xhtml
new file mode 100644
index 0000000000..6443c22d6c
--- /dev/null
+++ b/dom/xul/crashtests/344215-1.xhtml
@@ -0,0 +1,7 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+<observes/>
+
+<description>You should not see any assertions in a debug build.</description>
+
+</window>
\ No newline at end of file
diff --git a/dom/xul/crashtests/354611-1.html b/dom/xul/crashtests/354611-1.html
new file mode 100644
index 0000000000..fe25de3660
--- /dev/null
+++ b/dom/xul/crashtests/354611-1.html
@@ -0,0 +1,20 @@
+<html>
+<head>
+<script>
+
+var XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+function boom()
+{
+  var z = document.createElementNS(XUL_NS, "window");
+  document.body.appendChild(z);
+  z.setAttribute("hidechrome", "true");
+}
+
+</script>
+
+<body onload="boom();">
+
+</body>
+
+</html>
\ No newline at end of file
diff --git a/dom/xul/crashtests/360078-1.xhtml b/dom/xul/crashtests/360078-1.xhtml
new file mode 100644
index 0000000000..a29087014d
--- /dev/null
+++ b/dom/xul/crashtests/360078-1.xhtml
@@ -0,0 +1,42 @@
+<html xmlns="http://www.w3.org/1999/xhtml" class="reftest-wait">
+<head>
+
+<style>
+<![CDATA[
+#baz { -moz-binding: url(360078-1xbl.xml#foo); }
+]]>
+</style>
+
+<script>
+<![CDATA[
+
+function stuff()
+{
+  var baz = document.getElementById("baz");
+  var count = 0;
+  
+  setTimeout(step, 30);
+  
+  function step()
+  {
+    ++count;
+    if (count < 15) {
+      baz.cloneNode(true);
+      setTimeout(step, 30);
+    }
+    else {
+      document.documentElement.removeAttribute("class");
+    }
+  }
+  
+}
+
+]]>
+</script>
+</head>
+<body onload="stuff()">
+
+<hbox xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" id="baz"></hbox>
+
+</body>
+</html>
diff --git a/dom/xul/crashtests/363791-1.xhtml b/dom/xul/crashtests/363791-1.xhtml
new file mode 100644
index 0000000000..0786960f23
--- /dev/null
+++ b/dom/xul/crashtests/363791-1.xhtml
@@ -0,0 +1,44 @@
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="setTimeout(boom, 30);" class="reftest-wait">
+
+<script>
+
+function boom()
+{
+  var tree = document.getElementById("tree");
+  var treecols = document.getElementById("treecols");
+  var treechildren = document.getElementById("treechildren");
+
+  tree.appendChild(treechildren); // no real change
+  
+  function boom2() {
+    treecols.parentNode.removeChild(treecols);
+    document.documentElement.removeAttribute("class");
+  }
+
+  setTimeout(boom2, 30);
+}
+
+</script>
+
+
+<tree rows="6" id="tree">
+
+  <treecols id="treecols">
+      <treecol id="firstname" label="First Name"/>
+  </treecols>
+
+  <treechildren id="treechildren">
+    <treeitem>
+      <treerow>
+        <treecell label="Bob"/>
+      </treerow>
+    </treeitem>
+  </treechildren>
+
+</tree>
+
+
+</window>
diff --git a/dom/xul/crashtests/384877-1-inner.xhtml b/dom/xul/crashtests/384877-1-inner.xhtml
new file mode 100644
index 0000000000..bd75aa87a7
--- /dev/null
+++ b/dom/xul/crashtests/384877-1-inner.xhtml
@@ -0,0 +1,12 @@
+<menupopup xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" id="d" popup="d">
+<script>
+function doe() {
+var d = document.getElementById('d');
+if (d.openPopup) {
+  d.openPopup(document.documentElement, 'before_start', 0, 0, false, false);
+//  alert(d.state);
+}
+  
+setTimeout(doe, 200);
+</script>
+</menupopup>
diff --git a/dom/xul/crashtests/384877-1.html b/dom/xul/crashtests/384877-1.html
new file mode 100644
index 0000000000..b4ecd1b5f8
--- /dev/null
+++ b/dom/xul/crashtests/384877-1.html
@@ -0,0 +1,9 @@
+<html class="reftest-wait">
+<head>
+<script>
+setTimeout('document.documentElement.className = ""', 1000);
+</script>
+<body>
+<iframe src="384877-1-inner.xhtml"></iframe>
+</body>
+</html>
diff --git a/dom/xul/crashtests/386914-1-inner.xhtml b/dom/xul/crashtests/386914-1-inner.xhtml
new file mode 100644
index 0000000000..909889d8b5
--- /dev/null
+++ b/dom/xul/crashtests/386914-1-inner.xhtml
@@ -0,0 +1,10 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" id="a">
+
+<box id="b" observes="a"/>
+
+<html:script xmlns:html="http://www.w3.org/1999/xhtml">
+document.getElementById('b').addEventListener('DOMAttrModified', function(e) {document.removeChild(document.documentElement);}, true);
+setTimeout(function() {document.getElementById('a').setAttribute('tabindex', '1') ;}, 100);
+</html:script>
+
+</window>
\ No newline at end of file
diff --git a/dom/xul/crashtests/386914-1.html b/dom/xul/crashtests/386914-1.html
new file mode 100644
index 0000000000..2bb1224ab7
--- /dev/null
+++ b/dom/xul/crashtests/386914-1.html
@@ -0,0 +1,9 @@
+<html class="reftest-wait">
+<head>
+<script>
+setTimeout('document.documentElement.className = ""', 1000);
+</script>
+<body>
+<iframe src="386914-1-inner.xhtml"></iframe>
+</body>
+</html>
diff --git a/dom/xul/crashtests/425821-1.xhtml b/dom/xul/crashtests/425821-1.xhtml
new file mode 100644
index 0000000000..9764e64aad
--- /dev/null
+++ b/dom/xul/crashtests/425821-1.xhtml
@@ -0,0 +1,15 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"  onload="boom();">
+<script type="text/javascript">
+
+function boom()
+{
+  var XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+  var wiz = document.createElementNS(XUL_NS, "wizard");
+  var btn = document.createElementNS(XUL_NS, "hbox");
+  btn.setAttribute("anonid", "Buttons");
+  wiz.appendChild(btn);
+  wiz.cloneNode(true);
+}
+
+</script>
+</window>
diff --git a/dom/xul/crashtests/428951-1.xhtml b/dom/xul/crashtests/428951-1.xhtml
new file mode 100644
index 0000000000..41dd353f3d
--- /dev/null
+++ b/dom/xul/crashtests/428951-1.xhtml
@@ -0,0 +1,21 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:mathml="http://www.w3.org/1998/Math/MathML">
+<box>
+  <box style="background: initial;" id="f">
+    <box style="margin-top: -9999999px;"/>
+  </box>
+  <mathml:divergence>
+    <box/>
+  </mathml:divergence>
+  <mathml:moment command="f"/>
+</box>
+
+<script id="script" xmlns="http://www.w3.org/1999/xhtml"><![CDATA[
+function init() {
+    var f = document.getElementsByTagName('mathml:divergence')[0];
+    window.addEventListener('DOMAttrModified',function() { f.remove();}, true);
+    var x=document.getElementsByTagName('mathml:moment')[0];
+    x.remove();
+}
+window.addEventListener("load", init, false);
+]]></script>
+</window>
diff --git a/dom/xul/crashtests/431906-1-inner.xhtml b/dom/xul/crashtests/431906-1-inner.xhtml
new file mode 100644
index 0000000000..367f621b2f
--- /dev/null
+++ b/dom/xul/crashtests/431906-1-inner.xhtml
@@ -0,0 +1,19 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<colgroup id="a" command="a">
+<box xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<box id="a" command="a"/>
+<box id="a" command="a"/>
+</box>
+</colgroup>
+<script><![CDATA[
+function doe(){
+document.getElementsByTagName('*')[1].setAttribute('id','');
+document.getElementsByTagName('*')[1].setAttribute('xmlns', '');
+document.getElementsByTagName('*')[3].setAttribute('id','');
+
+document.getElementsByTagName('*')[4].removeAttribute('xmlns');
+document.getElementsByTagName('*')[4].setAttribute('width', '1px');
+}
+setTimeout(doe,100);
+]]></script>
+</html>
\ No newline at end of file
diff --git a/dom/xul/crashtests/431906-1.html b/dom/xul/crashtests/431906-1.html
new file mode 100644
index 0000000000..a8570b1bc0
--- /dev/null
+++ b/dom/xul/crashtests/431906-1.html
@@ -0,0 +1,9 @@
+<html class="reftest-wait">
+<head>
+<script>
+setTimeout('document.documentElement.className = ""', 1000);
+</script>
+<body>
+<iframe src="431906-1-inner.xhtml"></iframe>
+</body>
+</html>
diff --git a/dom/xul/crashtests/461917-1.xhtml b/dom/xul/crashtests/461917-1.xhtml
new file mode 100644
index 0000000000..15792f6f0f
--- /dev/null
+++ b/dom/xul/crashtests/461917-1.xhtml
@@ -0,0 +1,6 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head></head>
+<body>
+<tabs xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" onselect="#"><wizard/></tabs>
+</body>
+</html>
diff --git a/dom/xul/crashtests/crashtests.list b/dom/xul/crashtests/crashtests.list
new file mode 100644
index 0000000000..970d57b457
--- /dev/null
+++ b/dom/xul/crashtests/crashtests.list
@@ -0,0 +1,17 @@
+load 107518-1.xml
+load chrome://reftest/content/crashtests/dom/xul/crashtests/253479-1.xhtml
+load chrome://reftest/content/crashtests/dom/xul/crashtests/253479-2.xhtml
+load chrome://reftest/content/crashtests/dom/xul/crashtests/326204-1.xhtml
+load 326644-1.html
+load chrome://reftest/content/crashtests/dom/xul/crashtests/326875-1.xhtml
+load 329982-1.xhtml
+load 336096-1.xhtml
+load chrome://reftest/content/crashtests/dom/xul/crashtests/344215-1.xhtml
+load 354611-1.html
+skip-if(Android) load chrome://reftest/content/crashtests/dom/xul/crashtests/363791-1.xhtml
+load 384877-1.html
+load 386914-1.html
+load chrome://reftest/content/crashtests/dom/xul/crashtests/425821-1.xhtml
+load chrome://reftest/content/crashtests/dom/xul/crashtests/428951-1.xhtml
+load 431906-1.html
+load 461917-1.xhtml
diff --git a/dom/xul/moz.build b/dom/xul/moz.build
new file mode 100644
index 0000000000..cbcf964a2b
--- /dev/null
+++ b/dom/xul/moz.build
@@ -0,0 +1,94 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Core", "XUL")
+
+if CONFIG["MOZ_BUILD_APP"] == "browser":
+    DEFINES["MOZ_BREAK_XUL_OVERLAYS"] = True
+
+MOCHITEST_CHROME_MANIFESTS += ["test/chrome.toml"]
+MOCHITEST_MANIFESTS += ["test/mochitest.toml"]
+
+EXPORTS += [
+    "nsXULCommandDispatcher.h",
+    "nsXULElement.h",
+    "nsXULPrototypeDocument.h",
+    "nsXULSortService.h",
+]
+
+EXPORTS.mozilla.dom += [
+    "XULBroadcastManager.h",
+    "XULButtonElement.h",
+    "XULFrameElement.h",
+    "XULMenuBarElement.h",
+    "XULMenuElement.h",
+    "XULMenuParentElement.h",
+    "XULPersist.h",
+    "XULPopupElement.h",
+    "XULResizerElement.h",
+    "XULTextElement.h",
+    "XULTooltipElement.h",
+    "XULTreeElement.h",
+]
+
+UNIFIED_SOURCES += [
+    "MenuBarListener.cpp",
+    "nsXULCommandDispatcher.cpp",
+    "nsXULContentSink.cpp",
+    "nsXULContentUtils.cpp",
+    "nsXULElement.cpp",
+    "nsXULPopupListener.cpp",
+    "nsXULPrototypeCache.cpp",
+    "nsXULPrototypeDocument.cpp",
+    "nsXULSortService.cpp",
+    "XULBroadcastManager.cpp",
+    "XULButtonElement.cpp",
+    "XULFrameElement.cpp",
+    "XULMenuBarElement.cpp",
+    "XULMenuElement.cpp",
+    "XULMenuParentElement.cpp",
+    "XULPersist.cpp",
+    "XULPopupElement.cpp",
+    "XULResizerElement.cpp",
+    "XULTextElement.cpp",
+    "XULTooltipElement.cpp",
+    "XULTreeElement.cpp",
+]
+
+XPIDL_SOURCES += [
+    "nsIBrowserController.idl",
+    "nsIController.idl",
+    "nsIControllers.idl",
+]
+
+XPIDL_MODULE = "xul"
+
+EXPORTS.mozilla.dom += [
+    "ChromeObserver.h",
+]
+
+
+UNIFIED_SOURCES += [
+    "ChromeObserver.cpp",
+    "nsXULControllers.cpp",
+]
+
+LOCAL_INCLUDES += [
+    "/docshell/base",
+    "/dom/base",
+    "/dom/html",
+    "/dom/xml",
+    "/layout/base",
+    "/layout/generic",
+    "/layout/style",
+    "/layout/xul",
+    "/layout/xul/tree",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
diff --git a/dom/xul/nsIBrowserController.idl b/dom/xul/nsIBrowserController.idl
new file mode 100644
index 0000000000..5a7b77529b
--- /dev/null
+++ b/dom/xul/nsIBrowserController.idl
@@ -0,0 +1,20 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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"
+
+/**
+ * This interface is used to accompany the nsIController for a
+ * <browser> element. It is used to update the commands in the
+ * parent process when the set of child command have changed.
+ */
+[scriptable, uuid(5bb3d56b-e733-4a2c-8a53-058123df65e2)]
+interface nsIBrowserController : nsISupports
+{
+  // Update the commands for a given action in the parent process.
+  void enableDisableCommands(in AString action,
+                             in Array<ACString> enabledCommands,
+                             in Array<ACString> disabledCommands);
+};
diff --git a/dom/xul/nsIController.idl b/dom/xul/nsIController.idl
new file mode 100644
index 0000000000..c3249c5749
--- /dev/null
+++ b/dom/xul/nsIController.idl
@@ -0,0 +1,39 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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"
+
+[scriptable, uuid(D5B61B82-1DA4-11d3-BF87-00105A1B0627)]
+interface nsIController : nsISupports {
+  boolean isCommandEnabled(in string command);
+  boolean supportsCommand(in string command);
+
+  [can_run_script]
+  void doCommand(in string command);
+
+  void onEvent(in string eventName);
+};
+
+
+/*
+
+  Enhanced controller interface that allows for passing of parameters
+  to commands.
+
+*/
+
+interface nsICommandParams;
+
+[scriptable, uuid(EEC0B435-7F53-44FE-B00A-CF3EED65C01A)]
+interface nsICommandController : nsISupports
+{
+
+  void        getCommandStateWithParams( in string command, in nsICommandParams aCommandParams);
+
+  [can_run_script]
+  void        doCommandWithParams(in string command, in nsICommandParams aCommandParams);
+
+  Array<ACString> getSupportedCommands();
+};
diff --git a/dom/xul/nsIControllers.idl b/dom/xul/nsIControllers.idl
new file mode 100644
index 0000000000..2f272faec1
--- /dev/null
+++ b/dom/xul/nsIControllers.idl
@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 nsIController;
+interface nsIDOMXULCommandDispatcher;
+
+[scriptable, uuid(f36e3ec1-9197-4ad8-8d4c-d3b1927fd6df)]
+interface nsIControllers : nsISupports
+{
+    nsIController getControllerForCommand(in string command);
+
+    void insertControllerAt(in unsigned long index, in nsIController controller);
+    nsIController removeControllerAt(in unsigned long index);
+    nsIController getControllerAt(in unsigned long index);
+
+    void appendController(in nsIController controller);
+    void removeController(in nsIController controller);
+
+    /*
+        Return an ID for this controller which is unique to this
+        nsIControllers.
+    */
+    unsigned long getControllerId(in nsIController controller);
+    /*
+        Get the controller specified by the given ID.
+    */
+    nsIController getControllerById(in unsigned long controllerID);
+
+    unsigned long getControllerCount();
+};
diff --git a/dom/xul/nsXULCommandDispatcher.cpp b/dom/xul/nsXULCommandDispatcher.cpp
new file mode 100644
index 0000000000..0b07086914
--- /dev/null
+++ b/dom/xul/nsXULCommandDispatcher.cpp
@@ -0,0 +1,435 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 sw=2 et tw=80: */
+/* 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/. */
+
+/*
+
+  This file provides the implementation for the XUL Command Dispatcher.
+
+ */
+
+#include "nsIContent.h"
+#include "nsFocusManager.h"
+#include "nsIControllers.h"
+#include "mozilla/dom/Document.h"
+#include "nsPresContext.h"
+#include "nsIScriptGlobalObject.h"
+#include "nsPIDOMWindow.h"
+#include "nsPIWindowRoot.h"
+#include "nsXULCommandDispatcher.h"
+#include "mozilla/Logging.h"
+#include "nsContentUtils.h"
+#include "nsReadableUtils.h"
+#include "nsCRT.h"
+#include "nsError.h"
+#include "mozilla/BasicEvents.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/ElementBinding.h"
+
+using namespace mozilla;
+using mozilla::dom::Document;
+using mozilla::dom::Element;
+
+#ifdef DEBUG
+static LazyLogModule gCommandLog("nsXULCommandDispatcher");
+#endif
+
+////////////////////////////////////////////////////////////////////////
+
+nsXULCommandDispatcher::nsXULCommandDispatcher(Document* aDocument)
+    : mDocument(aDocument), mUpdaters(nullptr), mLocked(false) {}
+
+nsXULCommandDispatcher::~nsXULCommandDispatcher() { Disconnect(); }
+
+// QueryInterface implementation for nsXULCommandDispatcher
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsXULCommandDispatcher)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMXULCommandDispatcher)
+  NS_INTERFACE_MAP_ENTRY(nsISupportsWeakReference)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIDOMXULCommandDispatcher)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsXULCommandDispatcher)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsXULCommandDispatcher)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsXULCommandDispatcher)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsXULCommandDispatcher)
+  tmp->Disconnect();
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_REFERENCE
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsXULCommandDispatcher)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDocument)
+  Updater* updater = tmp->mUpdaters;
+  while (updater) {
+    cb.NoteXPCOMChild(updater->mElement);
+    updater = updater->mNext;
+  }
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+void nsXULCommandDispatcher::Disconnect() {
+  while (mUpdaters) {
+    Updater* doomed = mUpdaters;
+    mUpdaters = mUpdaters->mNext;
+    delete doomed;
+  }
+  mDocument = nullptr;
+}
+
+already_AddRefed<nsPIWindowRoot> nsXULCommandDispatcher::GetWindowRoot() {
+  if (mDocument) {
+    if (nsCOMPtr<nsPIDOMWindowOuter> window = mDocument->GetWindow()) {
+      return window->GetTopWindowRoot();
+    }
+  }
+
+  return nullptr;
+}
+
+Element* nsXULCommandDispatcher::GetRootFocusedContentAndWindow(
+    nsPIDOMWindowOuter** aWindow) {
+  *aWindow = nullptr;
+
+  if (!mDocument) {
+    return nullptr;
+  }
+
+  if (nsCOMPtr<nsPIDOMWindowOuter> win = mDocument->GetWindow()) {
+    if (nsCOMPtr<nsPIDOMWindowOuter> rootWindow = win->GetPrivateRoot()) {
+      return nsFocusManager::GetFocusedDescendant(
+          rootWindow, nsFocusManager::eIncludeAllDescendants, aWindow);
+    }
+  }
+
+  return nullptr;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::GetFocusedElement(Element** aElement) {
+  *aElement = nullptr;
+
+  nsCOMPtr<nsPIDOMWindowOuter> focusedWindow;
+  RefPtr<Element> focusedContent =
+      GetRootFocusedContentAndWindow(getter_AddRefs(focusedWindow));
+  if (focusedContent) {
+    // Make sure the caller can access the focused element.
+    if (!nsContentUtils::SubjectPrincipalOrSystemIfNativeCaller()->Subsumes(
+            focusedContent->NodePrincipal())) {
+      // XXX This might want to return null, but we use that return value
+      // to mean "there is no focused element," so to be clear, throw an
+      // exception.
+      return NS_ERROR_DOM_SECURITY_ERR;
+    }
+  }
+
+  focusedContent.forget(aElement);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::GetFocusedWindow(mozIDOMWindowProxy** aWindow) {
+  *aWindow = nullptr;
+
+  nsCOMPtr<nsPIDOMWindowOuter> window;
+  GetRootFocusedContentAndWindow(getter_AddRefs(window));
+  if (!window) return NS_OK;
+
+  // Make sure the caller can access this window. The caller can access this
+  // window iff it can access the document.
+  nsCOMPtr<Document> doc = window->GetDoc();
+
+  // Note: If there is no document, then this window has been cleared and
+  // there's nothing left to protect, so let the window pass through.
+  if (doc && !nsContentUtils::CanCallerAccess(doc))
+    return NS_ERROR_DOM_SECURITY_ERR;
+
+  window.forget(aWindow);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::SetFocusedElement(Element* aElement) {
+  RefPtr<nsFocusManager> fm = nsFocusManager::GetFocusManager();
+  NS_ENSURE_TRUE(fm, NS_ERROR_FAILURE);
+
+  if (aElement) {
+    return fm->SetFocus(aElement, 0);
+  }
+
+  // if aElement is null, clear the focus in the currently focused child window
+  nsCOMPtr<nsPIDOMWindowOuter> focusedWindow;
+  GetRootFocusedContentAndWindow(getter_AddRefs(focusedWindow));
+  return fm->ClearFocus(focusedWindow);
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::SetFocusedWindow(mozIDOMWindowProxy* aWindow) {
+  NS_ENSURE_TRUE(aWindow, NS_OK);  // do nothing if set to null
+
+  nsCOMPtr<nsPIDOMWindowOuter> window = nsPIDOMWindowOuter::From(aWindow);
+  NS_ENSURE_TRUE(window, NS_ERROR_FAILURE);
+
+  RefPtr<nsFocusManager> fm = nsFocusManager::GetFocusManager();
+  NS_ENSURE_TRUE(fm, NS_ERROR_FAILURE);
+
+  // get the containing frame for the window, and set it as focused. This will
+  // end up focusing whatever is currently focused inside the frame. Since
+  // setting the command dispatcher's focused window doesn't raise the window,
+  // setting it to a top-level window doesn't need to do anything.
+  RefPtr<Element> frameElement = window->GetFrameElementInternal();
+  if (frameElement) {
+    return fm->SetFocus(frameElement, 0);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::AdvanceFocus() {
+  return AdvanceFocusIntoSubtree(nullptr);
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::AdvanceFocusIntoSubtree(Element* aElt) {
+  return MoveFocusIntoSubtree(aElt, /* aForward = */ true);
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::RewindFocus() {
+  return MoveFocusIntoSubtree(nullptr, /* aForward = */ false);
+}
+
+nsresult nsXULCommandDispatcher::MoveFocusIntoSubtree(Element* aElt,
+                                                      bool aForward) {
+  nsCOMPtr<nsPIDOMWindowOuter> win;
+  GetRootFocusedContentAndWindow(getter_AddRefs(win));
+
+  RefPtr<Element> result;
+  nsFocusManager* fm = nsFocusManager::GetFocusManager();
+  if (!fm) {
+    return NS_OK;
+  }
+  auto flags = nsFocusManager::ProgrammaticFocusFlags(dom::FocusOptions()) |
+               nsIFocusManager::FLAG_BYMOVEFOCUS;
+  auto type = aForward ? nsIFocusManager::MOVEFOCUS_FORWARD
+                       : nsIFocusManager::MOVEFOCUS_BACKWARD;
+  return fm->MoveFocus(win, aElt, type, flags, getter_AddRefs(result));
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::AddCommandUpdater(Element* aElement,
+                                          const nsAString& aEvents,
+                                          const nsAString& aTargets) {
+  MOZ_ASSERT(aElement != nullptr, "null ptr");
+  if (!aElement) return NS_ERROR_NULL_POINTER;
+
+  NS_ENSURE_TRUE(mDocument, NS_ERROR_UNEXPECTED);
+
+  nsresult rv = nsContentUtils::CheckSameOrigin(mDocument, aElement);
+
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  Updater* updater = mUpdaters;
+  Updater** link = &mUpdaters;
+
+  while (updater) {
+    if (updater->mElement == aElement) {
+#ifdef DEBUG
+      if (MOZ_LOG_TEST(gCommandLog, LogLevel::Debug)) {
+        nsAutoCString eventsC, targetsC, aeventsC, atargetsC;
+        LossyCopyUTF16toASCII(updater->mEvents, eventsC);
+        LossyCopyUTF16toASCII(updater->mTargets, targetsC);
+        CopyUTF16toUTF8(aEvents, aeventsC);
+        CopyUTF16toUTF8(aTargets, atargetsC);
+        MOZ_LOG(gCommandLog, LogLevel::Debug,
+                ("xulcmd[%p] replace %p(events=%s targets=%s) with (events=%s "
+                 "targets=%s)",
+                 this, aElement, eventsC.get(), targetsC.get(), aeventsC.get(),
+                 atargetsC.get()));
+      }
+#endif
+
+      // If the updater was already in the list, then replace
+      // (?) the 'events' and 'targets' filters with the new
+      // specification.
+      updater->mEvents = aEvents;
+      updater->mTargets = aTargets;
+      return NS_OK;
+    }
+
+    link = &(updater->mNext);
+    updater = updater->mNext;
+  }
+#ifdef DEBUG
+  if (MOZ_LOG_TEST(gCommandLog, LogLevel::Debug)) {
+    nsAutoCString aeventsC, atargetsC;
+    CopyUTF16toUTF8(aEvents, aeventsC);
+    CopyUTF16toUTF8(aTargets, atargetsC);
+
+    MOZ_LOG(gCommandLog, LogLevel::Debug,
+            ("xulcmd[%p] add     %p(events=%s targets=%s)", this, aElement,
+             aeventsC.get(), atargetsC.get()));
+  }
+#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(Element* aElement) {
+  MOZ_ASSERT(aElement != nullptr, "null ptr");
+  if (!aElement) return NS_ERROR_NULL_POINTER;
+
+  Updater* updater = mUpdaters;
+  Updater** link = &mUpdaters;
+
+  while (updater) {
+    if (updater->mElement == aElement) {
+#ifdef DEBUG
+      if (MOZ_LOG_TEST(gCommandLog, LogLevel::Debug)) {
+        nsAutoCString eventsC, targetsC;
+        LossyCopyUTF16toASCII(updater->mEvents, eventsC);
+        LossyCopyUTF16toASCII(updater->mTargets, targetsC);
+        MOZ_LOG(gCommandLog, LogLevel::Debug,
+                ("xulcmd[%p] remove  %p(events=%s targets=%s)", this, aElement,
+                 eventsC.get(), targetsC.get()));
+      }
+#endif
+
+      *link = updater->mNext;
+      delete updater;
+      return NS_OK;
+    }
+
+    link = &(updater->mNext);
+    updater = updater->mNext;
+  }
+
+  // Hmm. Not found. Oh well.
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::UpdateCommands(const nsAString& aEventName) {
+  if (mLocked) {
+    if (!mPendingUpdates.Contains(aEventName)) {
+      mPendingUpdates.AppendElement(aEventName);
+    }
+
+    return NS_OK;
+  }
+
+  nsAutoString id;
+  RefPtr<Element> element;
+  GetFocusedElement(getter_AddRefs(element));
+  if (element) {
+    element->GetAttr(nsGkAtoms::id, id);
+  }
+
+  nsCOMArray<nsIContent> updaters;
+
+  for (Updater* updater = mUpdaters; updater != nullptr;
+       updater = updater->mNext) {
+    // Skip any nodes that don't match our 'events' or 'targets'
+    // filters.
+    if (!Matches(updater->mEvents, aEventName)) continue;
+
+    if (!Matches(updater->mTargets, id)) continue;
+
+    nsIContent* content = updater->mElement;
+    NS_ASSERTION(content != nullptr, "mElement is null");
+    if (!content) return NS_ERROR_UNEXPECTED;
+
+    updaters.AppendObject(content);
+  }
+
+  for (nsIContent* content : updaters) {
+#ifdef DEBUG
+    if (MOZ_LOG_TEST(gCommandLog, LogLevel::Debug)) {
+      nsAutoCString aeventnameC;
+      CopyUTF16toUTF8(aEventName, aeventnameC);
+      MOZ_LOG(
+          gCommandLog, LogLevel::Debug,
+          ("xulcmd[%p] update %p event=%s", this, content, aeventnameC.get()));
+    }
+#endif
+
+    WidgetEvent event(true, eXULCommandUpdate);
+    EventDispatcher::Dispatch(MOZ_KnownLive(content), nullptr, &event);
+  }
+  return NS_OK;
+}
+
+bool nsXULCommandDispatcher::Matches(const nsString& aList,
+                                     const nsAString& aElement) {
+  if (aList.EqualsLiteral("*")) return true;  // match _everything_!
+
+  int32_t indx = aList.Find(PromiseFlatString(aElement));
+  if (indx == -1) return false;  // not in the list at all
+
+  // okay, now make sure it's not a substring snafu; e.g., 'ur'
+  // found inside of 'blur'.
+  if (indx > 0) {
+    char16_t ch = aList[indx - 1];
+    if (!nsCRT::IsAsciiSpace(ch) && ch != char16_t(',')) return false;
+  }
+
+  if (indx + aElement.Length() < aList.Length()) {
+    char16_t ch = aList[indx + aElement.Length()];
+    if (!nsCRT::IsAsciiSpace(ch) && ch != char16_t(',')) return false;
+  }
+
+  return true;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::GetControllers(nsIControllers** aResult) {
+  nsCOMPtr<nsPIWindowRoot> root = GetWindowRoot();
+  NS_ENSURE_TRUE(root, NS_ERROR_FAILURE);
+
+  return root->GetControllers(false /* for any window */, aResult);
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::GetControllerForCommand(const char* aCommand,
+                                                nsIController** _retval) {
+  nsCOMPtr<nsPIWindowRoot> root = GetWindowRoot();
+  NS_ENSURE_TRUE(root, NS_ERROR_FAILURE);
+
+  return root->GetControllerForCommand(aCommand, false /* for any window */,
+                                       _retval);
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::Lock() {
+  // Since locking is used only as a performance optimization, we don't worry
+  // about nested lock calls. If that does happen, it just means we will unlock
+  // and process updates earlier.
+  mLocked = true;
+  return NS_OK;
+}
+
+// TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHODIMP nsXULCommandDispatcher::Unlock() {
+  if (mLocked) {
+    mLocked = false;
+
+    // Handle any pending updates one at a time. In the unlikely case where a
+    // lock is added during the update, break out.
+    while (!mLocked && mPendingUpdates.Length() > 0) {
+      nsString name = mPendingUpdates.ElementAt(0);
+      mPendingUpdates.RemoveElementAt(0);
+      UpdateCommands(name);
+    }
+  }
+
+  return NS_OK;
+}
diff --git a/dom/xul/nsXULCommandDispatcher.h b/dom/xul/nsXULCommandDispatcher.h
new file mode 100644
index 0000000000..1f988c752e
--- /dev/null
+++ b/dom/xul/nsXULCommandDispatcher.h
@@ -0,0 +1,82 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+/*
+
+   This is the focus manager for XUL documents.
+
+*/
+
+#ifndef nsXULCommandDispatcher_h__
+#define nsXULCommandDispatcher_h__
+
+#include "nsCOMPtr.h"
+#include "nsIDOMXULCommandDispatcher.h"
+#include "nsWeakReference.h"
+#include "nsString.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsTArray.h"
+#include "mozilla/RefPtr.h"
+
+class nsPIDOMWindowOuter;
+class nsPIWindowRoot;
+
+namespace mozilla::dom {
+class Document;
+class Element;
+}  // namespace mozilla::dom
+
+class nsXULCommandDispatcher : public nsIDOMXULCommandDispatcher,
+                               public nsSupportsWeakReference {
+  using Document = mozilla::dom::Document;
+  using Element = mozilla::dom::Element;
+
+ public:
+  explicit nsXULCommandDispatcher(Document* aDocument);
+
+  // nsISupports
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(nsXULCommandDispatcher,
+                                           nsIDOMXULCommandDispatcher)
+
+  // nsIDOMXULCommandDispatcher interface
+  NS_DECL_NSIDOMXULCOMMANDDISPATCHER
+
+  void Disconnect();
+
+ protected:
+  virtual ~nsXULCommandDispatcher();
+
+  already_AddRefed<nsPIWindowRoot> GetWindowRoot();
+
+  Element* GetRootFocusedContentAndWindow(nsPIDOMWindowOuter** aWindow);
+  nsresult MoveFocusIntoSubtree(Element*, bool aForward);
+
+  RefPtr<Document> mDocument;
+
+  class Updater {
+   public:
+    Updater(Element* aElement, const nsAString& aEvents,
+            const nsAString& aTargets)
+        : mElement(aElement),
+          mEvents(aEvents),
+          mTargets(aTargets),
+          mNext(nullptr) {}
+
+    RefPtr<Element> mElement;
+    nsString mEvents;
+    nsString mTargets;
+    Updater* mNext;
+  };
+
+  Updater* mUpdaters;
+
+  bool Matches(const nsString& aList, const nsAString& aElement);
+
+  bool mLocked;
+  nsTArray<nsString> mPendingUpdates;
+};
+
+#endif  // nsXULCommandDispatcher_h__
diff --git a/dom/xul/nsXULContentSink.cpp b/dom/xul/nsXULContentSink.cpp
new file mode 100644
index 0000000000..197c5c2b3b
--- /dev/null
+++ b/dom/xul/nsXULContentSink.cpp
@@ -0,0 +1,861 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+/*
+ * An implementation for a Gecko-style content sink that knows how
+ * to build a content model (the "prototype" document) from XUL.
+ *
+ * For more information on XUL,
+ * see http://developer.mozilla.org/en/docs/XUL
+ */
+
+#include "nsXULContentSink.h"
+
+#include "jsfriendapi.h"
+
+#include "nsCOMPtr.h"
+#include "nsIContentSink.h"
+#include "mozilla/dom/Document.h"
+#include "nsIFormControl.h"
+#include "mozilla/dom/NodeInfo.h"
+#include "nsIScriptContext.h"
+#include "nsIScriptGlobalObject.h"
+#include "nsNameSpaceManager.h"
+#include "nsParserBase.h"
+#include "nsViewManager.h"
+#include "nsIScriptSecurityManager.h"
+#include "nsLayoutCID.h"
+#include "nsNetUtil.h"
+#include "nsString.h"
+#include "nsReadableUtils.h"
+#include "nsXULElement.h"
+#include "mozilla/Logging.h"
+#include "nsCRT.h"
+
+#include "nsXULPrototypeDocument.h"  // XXXbe temporary
+#include "mozilla/css/Loader.h"
+
+#include "nsUnicharUtils.h"
+#include "nsGkAtoms.h"
+#include "nsContentUtils.h"
+#include "nsAttrName.h"
+#include "nsXMLContentSink.h"
+#include "nsIScriptError.h"
+#include "nsContentTypeParser.h"
+
+static mozilla::LazyLogModule gContentSinkLog("nsXULContentSink");
+
+using namespace mozilla;
+using namespace mozilla::dom;
+//----------------------------------------------------------------------
+
+XULContentSinkImpl::ContextStack::ContextStack() : mTop(nullptr), mDepth(0) {}
+
+XULContentSinkImpl::ContextStack::~ContextStack() {
+  while (mTop) {
+    Entry* doomed = mTop;
+    mTop = mTop->mNext;
+    delete doomed;
+  }
+}
+
+void XULContentSinkImpl::ContextStack::Push(RefPtr<nsXULPrototypeNode>&& aNode,
+                                            State aState) {
+  mTop = new Entry(std::move(aNode), aState, mTop);
+  ++mDepth;
+}
+
+nsresult XULContentSinkImpl::ContextStack::Pop(State* aState) {
+  if (mDepth == 0) return NS_ERROR_UNEXPECTED;
+
+  Entry* entry = mTop;
+  mTop = mTop->mNext;
+  --mDepth;
+
+  *aState = entry->mState;
+  delete entry;
+
+  return NS_OK;
+}
+
+nsresult XULContentSinkImpl::ContextStack::GetTopNode(
+    RefPtr<nsXULPrototypeNode>& aNode) {
+  if (mDepth == 0) return NS_ERROR_UNEXPECTED;
+
+  aNode = mTop->mNode;
+  return NS_OK;
+}
+
+nsresult XULContentSinkImpl::ContextStack::GetTopChildren(
+    nsPrototypeArray** aChildren) {
+  if (mDepth == 0) return NS_ERROR_UNEXPECTED;
+
+  *aChildren = &(mTop->mChildren);
+  return NS_OK;
+}
+
+void XULContentSinkImpl::ContextStack::Clear() {
+  Entry* cur = mTop;
+  while (cur) {
+    // Release the root element (and its descendants).
+    Entry* next = cur->mNext;
+    delete cur;
+    cur = next;
+  }
+
+  mTop = nullptr;
+  mDepth = 0;
+}
+
+void XULContentSinkImpl::ContextStack::Traverse(
+    nsCycleCollectionTraversalCallback& aCb) {
+  nsCycleCollectionTraversalCallback& cb = aCb;
+  for (ContextStack::Entry* tmp = mTop; tmp; tmp = tmp->mNext) {
+    NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mNode)
+    NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mChildren)
+  }
+}
+
+//----------------------------------------------------------------------
+
+XULContentSinkImpl::XULContentSinkImpl()
+    : mText(nullptr),
+      mTextLength(0),
+      mTextSize(0),
+      mConstrainSize(true),
+      mState(eInProlog) {}
+
+XULContentSinkImpl::~XULContentSinkImpl() {
+  // The context stack _should_ be empty, unless something has gone wrong.
+  NS_ASSERTION(mContextStack.Depth() == 0, "Context stack not empty?");
+  mContextStack.Clear();
+
+  free(mText);
+}
+
+//----------------------------------------------------------------------
+// nsISupports interface
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(XULContentSinkImpl)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(XULContentSinkImpl)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mNodeInfoManager)
+  tmp->mContextStack.Clear();
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mPrototype)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mParser)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(XULContentSinkImpl)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mNodeInfoManager)
+  tmp->mContextStack.Traverse(cb);
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPrototype)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mParser)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(XULContentSinkImpl)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIXMLContentSink)
+  NS_INTERFACE_MAP_ENTRY(nsIXMLContentSink)
+  NS_INTERFACE_MAP_ENTRY(nsIExpatSink)
+  NS_INTERFACE_MAP_ENTRY(nsIContentSink)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(XULContentSinkImpl)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(XULContentSinkImpl)
+
+//----------------------------------------------------------------------
+// nsIContentSink interface
+
+NS_IMETHODIMP
+XULContentSinkImpl::DidBuildModel(bool aTerminated) {
+  nsCOMPtr<Document> doc(mDocument);
+  if (doc) {
+    mPrototype->NotifyLoadDone();
+    mDocument = nullptr;
+  }
+
+  // Drop our reference to the parser to get rid of a circular
+  // reference.
+  mParser = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::WillInterrupt(void) {
+  // XXX Notify the docshell, if necessary
+  return NS_OK;
+}
+
+void XULContentSinkImpl::WillResume() {
+  // XXX Notify the docshell, if necessary
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::SetParser(nsParserBase* aParser) {
+  mParser = aParser;
+  return NS_OK;
+}
+
+void XULContentSinkImpl::SetDocumentCharset(
+    NotNull<const Encoding*> aEncoding) {
+  nsCOMPtr<Document> doc(mDocument);
+  if (doc) {
+    doc->SetDocumentCharacterSet(aEncoding);
+  }
+}
+
+nsISupports* XULContentSinkImpl::GetTarget() { return ToSupports(mDocument); }
+
+//----------------------------------------------------------------------
+
+nsresult XULContentSinkImpl::Init(Document* aDocument,
+                                  nsXULPrototypeDocument* aPrototype) {
+  MOZ_ASSERT(aDocument != nullptr, "null ptr");
+  if (!aDocument) return NS_ERROR_NULL_POINTER;
+
+  mDocument = aDocument;
+  mPrototype = aPrototype;
+
+  mDocumentURL = mPrototype->GetURI();
+  mNodeInfoManager = aPrototype->GetNodeInfoManager();
+  if (!mNodeInfoManager) return NS_ERROR_UNEXPECTED;
+
+  mState = eInProlog;
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------
+//
+// Text buffering
+//
+
+bool XULContentSinkImpl::IsDataInBuffer(char16_t* buffer, int32_t length) {
+  for (int32_t i = 0; i < length; ++i) {
+    if (buffer[i] == ' ' || buffer[i] == '\t' || buffer[i] == '\n' ||
+        buffer[i] == '\r')
+      continue;
+
+    return true;
+  }
+  return false;
+}
+
+nsresult XULContentSinkImpl::FlushText(bool aCreateTextNode) {
+  nsresult rv;
+
+  do {
+    // Don't do anything if there's no text to create a node from, or
+    // if they've told us not to create a text node
+    if (!mTextLength) break;
+
+    if (!aCreateTextNode) break;
+
+    RefPtr<nsXULPrototypeNode> node;
+    rv = mContextStack.GetTopNode(node);
+    if (NS_FAILED(rv)) return rv;
+
+    bool stripWhitespace = false;
+    if (node->mType == nsXULPrototypeNode::eType_Element) {
+      mozilla::dom::NodeInfo* nodeInfo =
+          static_cast<nsXULPrototypeElement*>(node.get())->mNodeInfo;
+
+      if (nodeInfo->NamespaceEquals(kNameSpaceID_XUL))
+        stripWhitespace = !nodeInfo->Equals(nsGkAtoms::label) &&
+                          !nodeInfo->Equals(nsGkAtoms::description);
+    }
+
+    // Don't bother if there's nothing but whitespace.
+    if (stripWhitespace && !IsDataInBuffer(mText, mTextLength)) break;
+
+    // Don't bother if we're not in XUL document body
+    if (mState != eInDocumentElement || mContextStack.Depth() == 0) break;
+
+    RefPtr<nsXULPrototypeText> text = new nsXULPrototypeText();
+    text->mValue.Assign(mText, mTextLength);
+    if (stripWhitespace) text->mValue.Trim(" \t\n\r");
+
+    // hook it up
+    nsPrototypeArray* children = nullptr;
+    rv = mContextStack.GetTopChildren(&children);
+    if (NS_FAILED(rv)) return rv;
+
+    children->AppendElement(text.forget());
+  } while (0);
+
+  // Reset our text buffer
+  mTextLength = 0;
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------
+
+nsresult XULContentSinkImpl::NormalizeAttributeString(
+    const char16_t* aExpatName, nsAttrName& aName) {
+  int32_t nameSpaceID;
+  RefPtr<nsAtom> prefix, localName;
+  nsContentUtils::SplitExpatName(aExpatName, getter_AddRefs(prefix),
+                                 getter_AddRefs(localName), &nameSpaceID);
+
+  if (nameSpaceID == kNameSpaceID_None) {
+    aName.SetTo(localName);
+
+    return NS_OK;
+  }
+
+  RefPtr<mozilla::dom::NodeInfo> ni;
+  ni = mNodeInfoManager->GetNodeInfo(localName, prefix, nameSpaceID,
+                                     nsINode::ATTRIBUTE_NODE);
+  aName.SetTo(ni);
+
+  return NS_OK;
+}
+
+/**** BEGIN NEW APIs ****/
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleStartElement(const char16_t* aName,
+                                       const char16_t** aAtts,
+                                       uint32_t aAttsCount,
+                                       uint32_t aLineNumber,
+                                       uint32_t aColumnNumber) {
+  // XXX Hopefully the parser will flag this before we get here. If
+  // we're in the epilog, there should be no new elements
+  MOZ_ASSERT(mState != eInEpilog, "tag in XUL doc epilog");
+  MOZ_ASSERT(aAttsCount % 2 == 0, "incorrect aAttsCount");
+
+  // Adjust aAttsCount so it's the actual number of attributes
+  aAttsCount /= 2;
+
+  if (mState == eInEpilog) return NS_ERROR_UNEXPECTED;
+
+  if (mState != eInScript) {
+    FlushText();
+  }
+
+  int32_t nameSpaceID;
+  RefPtr<nsAtom> prefix, localName;
+  nsContentUtils::SplitExpatName(aName, getter_AddRefs(prefix),
+                                 getter_AddRefs(localName), &nameSpaceID);
+
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo;
+  nodeInfo = mNodeInfoManager->GetNodeInfo(localName, prefix, nameSpaceID,
+                                           nsINode::ELEMENT_NODE);
+
+  nsresult rv = NS_OK;
+  switch (mState) {
+    case eInProlog:
+      // We're the root document element
+      rv = OpenRoot(aAtts, aAttsCount, nodeInfo);
+      break;
+
+    case eInDocumentElement:
+      rv = OpenTag(aAtts, aAttsCount, aLineNumber, nodeInfo);
+      break;
+
+    case eInEpilog:
+    case eInScript:
+      MOZ_LOG(
+          gContentSinkLog, LogLevel::Warning,
+          ("xul: warning: unexpected tags in epilog at line %d", aLineNumber));
+      rv = NS_ERROR_UNEXPECTED;  // XXX
+      break;
+  }
+
+  return rv;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleEndElement(const char16_t* aName) {
+  // Never EVER return anything but NS_OK or
+  // NS_ERROR_HTMLPARSER_BLOCK from this method. Doing so will blow
+  // the parser's little mind all over the planet.
+  nsresult rv;
+
+  RefPtr<nsXULPrototypeNode> node;
+  rv = mContextStack.GetTopNode(node);
+
+  if (NS_FAILED(rv)) {
+    return NS_OK;
+  }
+
+  switch (node->mType) {
+    case nsXULPrototypeNode::eType_Element: {
+      // Flush any text _now_, so that we'll get text nodes created
+      // before popping the stack.
+      FlushText();
+
+      // Pop the context stack and do prototype hookup.
+      nsPrototypeArray* children = nullptr;
+      rv = mContextStack.GetTopChildren(&children);
+      if (NS_FAILED(rv)) return rv;
+
+      nsXULPrototypeElement* element =
+          static_cast<nsXULPrototypeElement*>(node.get());
+
+      int32_t count = children->Length();
+      if (count) {
+        element->mChildren.SetCapacity(count);
+
+        for (int32_t i = 0; i < count; ++i)
+          element->mChildren.AppendElement(children->ElementAt(i));
+      }
+    } break;
+
+    case nsXULPrototypeNode::eType_Script: {
+      nsXULPrototypeScript* script =
+          static_cast<nsXULPrototypeScript*>(node.get());
+
+      // If given a src= attribute, we must ignore script tag content.
+      if (!script->mSrcURI && !script->HasStencil()) {
+        nsCOMPtr<Document> doc(mDocument);
+
+        script->mOutOfLine = false;
+        if (doc) {
+          script->Compile(mText, mTextLength, mDocumentURL, script->mLineNo,
+                          doc);
+        }
+      }
+
+      FlushText(false);
+    } break;
+
+    default:
+      NS_ERROR("didn't expect that");
+      break;
+  }
+
+  rv = mContextStack.Pop(&mState);
+  NS_ASSERTION(NS_SUCCEEDED(rv), "context stack corrupted");
+  if (NS_FAILED(rv)) return rv;
+
+  if (mContextStack.Depth() == 0) {
+    // The root element should -always- be an element, because
+    // it'll have been created via XULContentSinkImpl::OpenRoot().
+    NS_ASSERTION(node->mType == nsXULPrototypeNode::eType_Element,
+                 "root is not an element");
+    if (node->mType != nsXULPrototypeNode::eType_Element)
+      return NS_ERROR_UNEXPECTED;
+
+    // Now that we're done parsing, set the prototype document's
+    // root element. This transfers ownership of the prototype
+    // element tree to the prototype document.
+    nsXULPrototypeElement* element =
+        static_cast<nsXULPrototypeElement*>(node.get());
+
+    mPrototype->SetRootElement(element);
+    mState = eInEpilog;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleComment(const char16_t* aName) {
+  FlushText();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleCDataSection(const char16_t* aData,
+                                       uint32_t aLength) {
+  FlushText();
+  return AddText(aData, aLength);
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleDoctypeDecl(const nsAString& aSubset,
+                                      const nsAString& aName,
+                                      const nsAString& aSystemId,
+                                      const nsAString& aPublicId,
+                                      nsISupports* aCatalogData) {
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleCharacterData(const char16_t* aData,
+                                        uint32_t aLength) {
+  if (aData && mState != eInProlog && mState != eInEpilog) {
+    return AddText(aData, aLength);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleProcessingInstruction(const char16_t* aTarget,
+                                                const char16_t* aData) {
+  FlushText();
+
+  const nsDependentString target(aTarget);
+  const nsDependentString data(aData);
+
+  // Note: the created nsXULPrototypePI has mRefCnt == 1
+  RefPtr<nsXULPrototypePI> pi = new nsXULPrototypePI();
+  pi->mTarget = target;
+  pi->mData = data;
+
+  if (mState == eInProlog) {
+    // Note: passing in already addrefed pi
+    return mPrototype->AddProcessingInstruction(pi);
+  }
+
+  nsresult rv;
+  nsPrototypeArray* children = nullptr;
+  rv = mContextStack.GetTopChildren(&children);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // XXX(Bug 1631371) Check if this should use a fallible operation as it
+  // pretended earlier.
+  children->AppendElement(pi);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::HandleXMLDeclaration(const char16_t* aVersion,
+                                         const char16_t* aEncoding,
+                                         int32_t aStandalone) {
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+XULContentSinkImpl::ReportError(const char16_t* aErrorText,
+                                const char16_t* aSourceText,
+                                nsIScriptError* aError, bool* _retval) {
+  MOZ_ASSERT(aError && aSourceText && aErrorText, "Check arguments!!!");
+
+  // The expat driver should report the error.
+  *_retval = true;
+
+  nsresult rv = NS_OK;
+
+  // make sure to empty the context stack so that
+  // <parsererror> could become the root element.
+  mContextStack.Clear();
+
+  mState = eInProlog;
+
+  // Clear any buffered-up text we have.  It's enough to set the length to 0.
+  // The buffer itself is allocated when we're created and deleted in our
+  // destructor, so don't mess with it.
+  mTextLength = 0;
+
+  // return leaving the document empty if we're asked to not add a <parsererror>
+  // root node
+  nsCOMPtr<Document> idoc(mDocument);
+  if (idoc && idoc->SuppressParserErrorElement()) {
+    return NS_OK;
+  };
+
+  const char16_t* noAtts[] = {0, 0};
+
+  constexpr auto errorNs =
+      u"http://www.mozilla.org/newlayout/xml/parsererror.xml"_ns;
+
+  nsAutoString parsererror(errorNs);
+  parsererror.Append((char16_t)0xFFFF);
+  parsererror.AppendLiteral("parsererror");
+
+  rv = HandleStartElement(parsererror.get(), noAtts, 0, 0, 0);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = HandleCharacterData(aErrorText, NS_strlen(aErrorText));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsAutoString sourcetext(errorNs);
+  sourcetext.Append((char16_t)0xFFFF);
+  sourcetext.AppendLiteral("sourcetext");
+
+  rv = HandleStartElement(sourcetext.get(), noAtts, 0, 0, 0);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = HandleCharacterData(aSourceText, NS_strlen(aSourceText));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = HandleEndElement(sourcetext.get());
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = HandleEndElement(parsererror.get());
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return rv;
+}
+
+nsresult XULContentSinkImpl::OpenRoot(const char16_t** aAttributes,
+                                      const uint32_t aAttrLen,
+                                      mozilla::dom::NodeInfo* aNodeInfo) {
+  NS_ASSERTION(mState == eInProlog, "how'd we get here?");
+  if (mState != eInProlog) return NS_ERROR_UNEXPECTED;
+
+  if (aNodeInfo->Equals(nsGkAtoms::script, kNameSpaceID_XHTML) ||
+      aNodeInfo->Equals(nsGkAtoms::script, kNameSpaceID_XUL)) {
+    MOZ_LOG(gContentSinkLog, LogLevel::Error,
+            ("xul: script tag not allowed as root content element"));
+
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // Create the element
+  RefPtr<nsXULPrototypeElement> element = new nsXULPrototypeElement(aNodeInfo);
+
+  // Add the attributes
+  nsresult rv = AddAttributes(aAttributes, aAttrLen, element);
+  if (NS_FAILED(rv)) return rv;
+
+  // Push the element onto the context stack, so that child
+  // containers will hook up to us as their parent.
+  mContextStack.Push(std::move(element), mState);
+
+  mState = eInDocumentElement;
+  return NS_OK;
+}
+
+nsresult XULContentSinkImpl::OpenTag(const char16_t** aAttributes,
+                                     const uint32_t aAttrLen,
+                                     const uint32_t aLineNumber,
+                                     mozilla::dom::NodeInfo* aNodeInfo) {
+  // Create the element
+  RefPtr<nsXULPrototypeElement> element = new nsXULPrototypeElement(aNodeInfo);
+
+  // Link this element to its parent.
+  nsPrototypeArray* children = nullptr;
+  nsresult rv = mContextStack.GetTopChildren(&children);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // Add the attributes
+  rv = AddAttributes(aAttributes, aAttrLen, element);
+  if (NS_FAILED(rv)) return rv;
+
+  children->AppendElement(element);
+
+  if (aNodeInfo->Equals(nsGkAtoms::script, kNameSpaceID_XHTML) ||
+      aNodeInfo->Equals(nsGkAtoms::script, kNameSpaceID_XUL)) {
+    // Do scripty things now
+    rv = OpenScript(aAttributes, aLineNumber);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    NS_ASSERTION(mState == eInScript || mState == eInDocumentElement,
+                 "Unexpected state");
+    if (mState == eInScript) {
+      // OpenScript has pushed the nsPrototypeScriptElement onto the
+      // stack, so we're done.
+      return NS_OK;
+    }
+  }
+
+  // Push the element onto the context stack, so that child
+  // containers will hook up to us as their parent.
+  mContextStack.Push(std::move(element), mState);
+
+  mState = eInDocumentElement;
+  return NS_OK;
+}
+
+nsresult XULContentSinkImpl::OpenScript(const char16_t** aAttributes,
+                                        const uint32_t aLineNumber) {
+  bool isJavaScript = true;
+  nsresult rv;
+
+  // Look for SRC attribute and look for a LANGUAGE attribute
+  nsAutoString src;
+  while (*aAttributes) {
+    const nsDependentString key(aAttributes[0]);
+    if (key.EqualsLiteral("src")) {
+      src.Assign(aAttributes[1]);
+    } else if (key.EqualsLiteral("type")) {
+      nsDependentString str(aAttributes[1]);
+      nsContentTypeParser parser(str);
+      nsAutoString mimeType;
+      rv = parser.GetType(mimeType);
+      if (NS_FAILED(rv)) {
+        if (rv == NS_ERROR_INVALID_ARG) {
+          // Fail immediately rather than checking if later things
+          // are okay.
+          return NS_OK;
+        }
+        // We do want the warning here
+        NS_ENSURE_SUCCESS(rv, rv);
+      }
+
+      // NOTE(emilio): Module scripts don't pass this test, aren't cached yet.
+      // If they become cached, then we need to tweak
+      // PrototypeDocumentContentSink and remove the special cases there.
+      if (nsContentUtils::IsJavascriptMIMEType(mimeType)) {
+        isJavaScript = true;
+
+        // Get the version string, and ensure that JavaScript supports it.
+        nsAutoString versionName;
+        rv = parser.GetParameter("version", versionName);
+
+        if (NS_SUCCEEDED(rv)) {
+          nsContentUtils::ReportToConsoleNonLocalized(
+              u"Versioned JavaScripts are no longer supported. "
+              "Please remove the version parameter."_ns,
+              nsIScriptError::errorFlag, "XUL Document"_ns, nullptr,
+              mDocumentURL, u""_ns, aLineNumber);
+          isJavaScript = false;
+        } else if (rv != NS_ERROR_INVALID_ARG) {
+          return rv;
+        }
+      } else {
+        isJavaScript = false;
+      }
+    } else if (key.EqualsLiteral("language")) {
+      // Language is deprecated, and the impl in ScriptLoader ignores the
+      // various version strings anyway.  So we make no attempt to support
+      // languages other than JS for language=
+      nsAutoString lang(aAttributes[1]);
+      if (nsContentUtils::IsJavaScriptLanguage(lang)) {
+        isJavaScript = true;
+      }
+    }
+    aAttributes += 2;
+  }
+
+  // Don't process scripts that aren't JavaScript.
+  if (!isJavaScript) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<Document> doc(mDocument);
+  nsCOMPtr<nsIScriptGlobalObject> globalObject;
+  if (doc) globalObject = do_QueryInterface(doc->GetWindow());
+  RefPtr<nsXULPrototypeScript> script = new nsXULPrototypeScript(aLineNumber);
+
+  // If there is a SRC attribute...
+  if (!src.IsEmpty()) {
+    // Use the SRC attribute value to load the URL
+    rv = NS_NewURI(getter_AddRefs(script->mSrcURI), src, nullptr, mDocumentURL);
+
+    // Check if this document is allowed to load a script from this source
+    // NOTE: if we ever allow scripts added via the DOM to run, we need to
+    // add a CheckLoadURI call for that as well.
+    if (NS_SUCCEEDED(rv)) {
+      if (!mSecMan)
+        mSecMan = do_GetService(NS_SCRIPTSECURITYMANAGER_CONTRACTID, &rv);
+      if (NS_SUCCEEDED(rv) && doc) {
+        rv = mSecMan->CheckLoadURIWithPrincipal(
+            doc->NodePrincipal(), script->mSrcURI,
+            nsIScriptSecurityManager::ALLOW_CHROME, doc->InnerWindowID());
+      }
+    }
+
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+
+    // Attempt to deserialize an out-of-line script from the FastLoad
+    // file right away.  Otherwise we'll end up reloading the script and
+    // corrupting the FastLoad file trying to serialize it, in the case
+    // where it's already there.
+    script->DeserializeOutOfLine(nullptr, mPrototype);
+  }
+
+  nsPrototypeArray* children = nullptr;
+  rv = mContextStack.GetTopChildren(&children);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  children->AppendElement(script);
+
+  mConstrainSize = false;
+
+  mContextStack.Push(script, mState);
+  mState = eInScript;
+
+  return NS_OK;
+}
+
+nsresult XULContentSinkImpl::AddAttributes(const char16_t** aAttributes,
+                                           const uint32_t aAttrLen,
+                                           nsXULPrototypeElement* aElement) {
+  // Add tag attributes to the element
+  nsresult rv;
+
+  // Create storage for the attributes
+  nsXULPrototypeAttribute* attrs = nullptr;
+  if (aAttrLen > 0) {
+    attrs = aElement->mAttributes.AppendElements(aAttrLen);
+  }
+
+  // Copy the attributes into the prototype
+  uint32_t i;
+  for (i = 0; i < aAttrLen; ++i) {
+    rv = NormalizeAttributeString(aAttributes[i * 2], attrs[i].mName);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = aElement->SetAttrAt(i, nsDependentString(aAttributes[i * 2 + 1]),
+                             mDocumentURL);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    if (MOZ_LOG_TEST(gContentSinkLog, LogLevel::Debug)) {
+      nsAutoString extraWhiteSpace;
+      int32_t cnt = mContextStack.Depth();
+      while (--cnt >= 0) extraWhiteSpace.AppendLiteral("  ");
+      nsAutoString qnameC, valueC;
+      qnameC.Assign(aAttributes[0]);
+      valueC.Assign(aAttributes[1]);
+      MOZ_LOG(gContentSinkLog, LogLevel::Debug,
+              ("xul: %.5d. %s    %s=%s",
+               -1,  // XXX pass in line number
+               NS_ConvertUTF16toUTF8(extraWhiteSpace).get(),
+               NS_ConvertUTF16toUTF8(qnameC).get(),
+               NS_ConvertUTF16toUTF8(valueC).get()));
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult XULContentSinkImpl::AddText(const char16_t* aText, int32_t aLength) {
+  // Create buffer when we first need it
+  if (0 == mTextSize) {
+    mText = (char16_t*)malloc(sizeof(char16_t) * 4096);
+    if (nullptr == mText) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+    mTextSize = 4096;
+  }
+
+  // Copy data from string into our buffer; flush buffer when it fills up
+  int32_t offset = 0;
+  while (0 != aLength) {
+    int32_t amount = mTextSize - mTextLength;
+    if (amount > aLength) {
+      amount = aLength;
+    }
+    if (0 == amount) {
+      if (mConstrainSize) {
+        nsresult rv = FlushText();
+        if (NS_OK != rv) {
+          return rv;
+        }
+      } else {
+        CheckedInt32 size = mTextSize;
+        size += aLength;
+        if (!size.isValid()) {
+          return NS_ERROR_OUT_OF_MEMORY;
+        }
+        mTextSize = size.value();
+
+        mText = (char16_t*)realloc(mText, sizeof(char16_t) * mTextSize);
+        if (nullptr == mText) {
+          return NS_ERROR_OUT_OF_MEMORY;
+        }
+      }
+    }
+    memcpy(&mText[mTextLength], aText + offset, sizeof(char16_t) * amount);
+
+    mTextLength += amount;
+    offset += amount;
+    aLength -= amount;
+  }
+
+  return NS_OK;
+}
diff --git a/dom/xul/nsXULContentSink.h b/dom/xul/nsXULContentSink.h
new file mode 100644
index 0000000000..63fa144156
--- /dev/null
+++ b/dom/xul/nsXULContentSink.h
@@ -0,0 +1,144 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+#ifndef nsXULContentSink_h__
+#define nsXULContentSink_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/WeakPtr.h"
+#include "nsIExpatSink.h"
+#include "nsIWeakReferenceUtils.h"
+#include "nsIXMLContentSink.h"
+#include "nsNodeInfoManager.h"
+#include "nsXULElement.h"
+#include "nsIDTD.h"
+
+class nsIScriptSecurityManager;
+class nsAttrName;
+class nsXULPrototypeDocument;
+class nsXULPrototypeElement;
+class nsXULPrototypeNode;
+
+class XULContentSinkImpl final : public nsIXMLContentSink, public nsIExpatSink {
+ public:
+  XULContentSinkImpl();
+
+  // nsISupports
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_NSIEXPATSINK
+
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(XULContentSinkImpl,
+                                           nsIXMLContentSink)
+
+  // nsIContentSink
+  NS_IMETHOD WillParse(void) override { return NS_OK; }
+  NS_IMETHOD DidBuildModel(bool aTerminated) override;
+  NS_IMETHOD WillInterrupt(void) override;
+  void WillResume() override;
+  NS_IMETHOD SetParser(nsParserBase* aParser) override;
+  virtual void FlushPendingNotifications(mozilla::FlushType aType) override {}
+  virtual void SetDocumentCharset(NotNull<const Encoding*> aEncoding) override;
+  virtual nsISupports* GetTarget() override;
+
+  /**
+   * Initialize the content sink, giving it a document with which to communicate
+   * with the outside world, and an nsXULPrototypeDocument to build.
+   */
+  nsresult Init(mozilla::dom::Document* aDocument,
+                nsXULPrototypeDocument* aPrototype);
+
+ protected:
+  virtual ~XULContentSinkImpl();
+
+  // pseudo-constants
+  char16_t* mText;
+  int32_t mTextLength;
+  int32_t mTextSize;
+  bool mConstrainSize;
+
+  nsresult AddAttributes(const char16_t** aAttributes, const uint32_t aAttrLen,
+                         nsXULPrototypeElement* aElement);
+
+  nsresult OpenRoot(const char16_t** aAttributes, const uint32_t aAttrLen,
+                    mozilla::dom::NodeInfo* aNodeInfo);
+
+  nsresult OpenTag(const char16_t** aAttributes, const uint32_t aAttrLen,
+                   const uint32_t aLineNumber,
+                   mozilla::dom::NodeInfo* aNodeInfo);
+
+  // If OpenScript returns NS_OK and after it returns our state is eInScript,
+  // that means that we created a prototype script and stuck it on
+  // mContextStack.  If NS_OK is returned but the state is still
+  // eInDocumentElement then we didn't create a prototype script (e.g. the
+  // script had an unknown type), and the caller should create a prototype
+  // element.
+  nsresult OpenScript(const char16_t** aAttributes, const uint32_t aLineNumber);
+
+  static bool IsDataInBuffer(char16_t* aBuffer, int32_t aLength);
+
+  // Text management
+  nsresult FlushText(bool aCreateTextNode = true);
+  nsresult AddText(const char16_t* aText, int32_t aLength);
+
+  RefPtr<nsNodeInfoManager> mNodeInfoManager;
+
+  nsresult NormalizeAttributeString(const char16_t* aExpatName,
+                                    nsAttrName& aName);
+
+ public:
+  enum State { eInProlog, eInDocumentElement, eInScript, eInEpilog };
+
+ protected:
+  State mState;
+
+  // content stack management
+  class ContextStack {
+   protected:
+    struct Entry {
+      RefPtr<nsXULPrototypeNode> mNode;
+      // a LOT of nodes have children; preallocate for 8
+      nsPrototypeArray mChildren;
+      State mState;
+      Entry* mNext;
+      Entry(RefPtr<nsXULPrototypeNode>&& aNode, State aState, Entry* aNext)
+          : mNode(std::move(aNode)),
+            mChildren(8),
+            mState(aState),
+            mNext(aNext) {}
+    };
+
+    Entry* mTop;
+    int32_t mDepth;
+
+   public:
+    ContextStack();
+    ~ContextStack();
+
+    int32_t Depth() { return mDepth; }
+
+    void Push(RefPtr<nsXULPrototypeNode>&& aNode, State aState);
+    nsresult Pop(State* aState);
+
+    nsresult GetTopNode(RefPtr<nsXULPrototypeNode>& aNode);
+    nsresult GetTopChildren(nsPrototypeArray** aChildren);
+
+    void Clear();
+
+    void Traverse(nsCycleCollectionTraversalCallback& aCallback);
+  };
+
+  friend class ContextStack;
+  ContextStack mContextStack;
+
+  mozilla::WeakPtr<mozilla::dom::Document> mDocument;
+  nsCOMPtr<nsIURI> mDocumentURL;  // [OWNER]
+
+  RefPtr<nsXULPrototypeDocument> mPrototype;  // [OWNER]
+
+  RefPtr<nsParserBase> mParser;
+  nsCOMPtr<nsIScriptSecurityManager> mSecMan;
+};
+
+#endif /* nsXULContentSink_h__ */
diff --git a/dom/xul/nsXULContentUtils.cpp b/dom/xul/nsXULContentUtils.cpp
new file mode 100644
index 0000000000..109aa1975a
--- /dev/null
+++ b/dom/xul/nsXULContentUtils.cpp
@@ -0,0 +1,89 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+/*
+
+  A package of routines shared by the XUL content code.
+
+ */
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/intl/LocaleService.h"
+#include "mozilla/intl/Collator.h"
+
+#include "nsCOMPtr.h"
+#include "nsComponentManagerUtils.h"
+#include "nsIContent.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "nsXULContentUtils.h"
+#include "nsLayoutCID.h"
+#include "nsString.h"
+#include "nsGkAtoms.h"
+
+using namespace mozilla;
+
+//------------------------------------------------------------------------
+
+const mozilla::intl::Collator* nsXULContentUtils::gCollator;
+
+//------------------------------------------------------------------------
+// Constructors n' stuff
+//
+
+nsresult nsXULContentUtils::Finish() {
+  if (gCollator) {
+    delete gCollator;
+    gCollator = nullptr;
+  }
+
+  return NS_OK;
+}
+
+const mozilla::intl::Collator* nsXULContentUtils::GetCollator() {
+  if (!gCollator) {
+    // Lazily initialize the Collator.
+    auto result = mozilla::intl::LocaleService::TryCreateComponent<
+        mozilla::intl::Collator>();
+    if (result.isErr()) {
+      NS_ERROR("couldn't create a mozilla::intl::Collator");
+      return nullptr;
+    }
+
+    auto collator = result.unwrap();
+
+    // Sort in a case-insensitive way, where "base" letters are considered
+    // equal, e.g: a = á, a = A, a ≠ b.
+    mozilla::intl::Collator::Options options{};
+    options.sensitivity = mozilla::intl::Collator::Sensitivity::Base;
+    auto optResult = collator->SetOptions(options);
+    if (optResult.isErr()) {
+      NS_ERROR("couldn't set options for mozilla::intl::Collator");
+      return nullptr;
+    }
+    gCollator = collator.release();
+  }
+
+  return gCollator;
+}
+
+//------------------------------------------------------------------------
+//
+
+nsresult nsXULContentUtils::FindChildByTag(nsIContent* aElement,
+                                           int32_t aNameSpaceID, nsAtom* aTag,
+                                           mozilla::dom::Element** aResult) {
+  for (nsIContent* child = aElement->GetFirstChild(); child;
+       child = child->GetNextSibling()) {
+    if (child->IsElement() && child->NodeInfo()->Equals(aTag, aNameSpaceID)) {
+      NS_ADDREF(*aResult = child->AsElement());
+      return NS_OK;
+    }
+  }
+
+  *aResult = nullptr;
+  return NS_RDF_NO_VALUE;  // not found
+}
diff --git a/dom/xul/nsXULContentUtils.h b/dom/xul/nsXULContentUtils.h
new file mode 100644
index 0000000000..c996bcec2b
--- /dev/null
+++ b/dom/xul/nsXULContentUtils.h
@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+/*
+
+  A package of routines shared by the XUL content code.
+
+ */
+
+#ifndef nsXULContentUtils_h__
+#define nsXULContentUtils_h__
+
+#include "nsISupports.h"
+
+class nsAtom;
+class nsIContent;
+
+namespace mozilla::dom {
+class Element;
+}
+namespace mozilla::intl {
+class Collator;
+}
+
+class nsXULContentUtils {
+ protected:
+  const static mozilla::intl::Collator* gCollator;
+
+  static bool gDisableXULCache;
+
+  static int DisableXULCacheChangedCallback(const char* aPrefName,
+                                            void* aClosure);
+
+ public:
+  static nsresult Finish();
+
+  static nsresult FindChildByTag(nsIContent* aElement, int32_t aNameSpaceID,
+                                 nsAtom* aTag, mozilla::dom::Element** aResult);
+
+  static const mozilla::intl::Collator* GetCollator();
+};
+
+#endif  // nsXULContentUtils_h__
diff --git a/dom/xul/nsXULControllers.cpp b/dom/xul/nsXULControllers.cpp
new file mode 100644
index 0000000000..3e96fd6e41
--- /dev/null
+++ b/dom/xul/nsXULControllers.cpp
@@ -0,0 +1,214 @@
+/* -*- 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/. */
+
+/*
+
+  This file provides the implementation for the XUL "controllers"
+  object.
+
+*/
+
+#include "nsString.h"
+
+#include "nsIControllers.h"
+#include "nsXULControllers.h"
+#include "nsIController.h"
+#include "mozilla/RefPtr.h"
+
+//----------------------------------------------------------------------
+
+nsXULControllerData::nsXULControllerData(uint32_t inControllerID,
+                                         nsIController* inController)
+    : mControllerID(inControllerID), mController(inController) {}
+
+nsresult nsXULControllerData::GetController(nsIController** outController) {
+  NS_IF_ADDREF(*outController = mController);
+  return NS_OK;
+}
+
+nsXULControllers::nsXULControllers() : mCurControllerID(0) {}
+
+nsXULControllers::~nsXULControllers(void) { DeleteControllers(); }
+
+void nsXULControllers::DeleteControllers() {
+  uint32_t count = mControllers.Length();
+  for (uint32_t i = 0; i < count; i++) {
+    nsXULControllerData* controllerData = mControllers.ElementAt(i);
+    delete controllerData;  // releases the nsIController
+  }
+
+  mControllers.Clear();
+}
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsXULControllers)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsXULControllers)
+  tmp->DeleteControllers();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsXULControllers)
+  {
+    uint32_t i, count = tmp->mControllers.Length();
+    for (i = 0; i < count; ++i) {
+      nsXULControllerData* controllerData = tmp->mControllers[i];
+      if (controllerData) {
+        cb.NoteXPCOMChild(controllerData->mController);
+      }
+    }
+  }
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsXULControllers)
+  NS_INTERFACE_MAP_ENTRY(nsIControllers)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIControllers)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsXULControllers)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsXULControllers)
+
+NS_IMETHODIMP
+nsXULControllers::GetControllerForCommand(const char* aCommand,
+                                          nsIController** _retval) {
+  NS_ENSURE_ARG_POINTER(_retval);
+  *_retval = nullptr;
+
+  uint32_t count = mControllers.Length();
+  for (uint32_t i = 0; i < count; i++) {
+    nsXULControllerData* controllerData = mControllers.ElementAt(i);
+    if (controllerData) {
+      nsCOMPtr<nsIController> controller;
+      controllerData->GetController(getter_AddRefs(controller));
+      if (controller) {
+        bool supportsCommand;
+        controller->SupportsCommand(aCommand, &supportsCommand);
+        if (supportsCommand) {
+          controller.forget(_retval);
+          return NS_OK;
+        }
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULControllers::InsertControllerAt(uint32_t aIndex,
+                                     nsIController* controller) {
+  nsXULControllerData* controllerData =
+      new nsXULControllerData(++mCurControllerID, controller);
+#ifdef DEBUG
+  nsXULControllerData** inserted =
+#endif
+      mControllers.InsertElementAt(aIndex, controllerData);
+  NS_ASSERTION(inserted != nullptr, "Insertion of controller failed");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULControllers::RemoveControllerAt(uint32_t aIndex, nsIController** _retval) {
+  NS_ENSURE_ARG_POINTER(_retval);
+  *_retval = nullptr;
+
+  nsXULControllerData* controllerData = mControllers.SafeElementAt(aIndex);
+  if (!controllerData) return NS_ERROR_FAILURE;
+
+  mControllers.RemoveElementAt(aIndex);
+
+  controllerData->GetController(_retval);
+  delete controllerData;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULControllers::GetControllerAt(uint32_t aIndex, nsIController** _retval) {
+  NS_ENSURE_ARG_POINTER(_retval);
+  *_retval = nullptr;
+
+  nsXULControllerData* controllerData = mControllers.SafeElementAt(aIndex);
+  if (!controllerData) return NS_ERROR_FAILURE;
+
+  return controllerData->GetController(_retval);  // does the addref
+}
+
+NS_IMETHODIMP
+nsXULControllers::AppendController(nsIController* controller) {
+  // This assigns controller IDs starting at 1 so we can use 0 to test if an ID
+  // was obtained
+  nsXULControllerData* controllerData =
+      new nsXULControllerData(++mCurControllerID, controller);
+
+#ifdef DEBUG
+  nsXULControllerData** appended =
+#endif
+      mControllers.AppendElement(controllerData);
+  NS_ASSERTION(appended != nullptr, "Appending controller failed");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULControllers::RemoveController(nsIController* controller) {
+  // first get the identity pointer
+  nsCOMPtr<nsISupports> controllerSup(do_QueryInterface(controller));
+  // then find it
+  uint32_t count = mControllers.Length();
+  for (uint32_t i = 0; i < count; i++) {
+    nsXULControllerData* controllerData = mControllers.ElementAt(i);
+    if (controllerData) {
+      nsCOMPtr<nsIController> thisController;
+      controllerData->GetController(getter_AddRefs(thisController));
+      nsCOMPtr<nsISupports> thisControllerSup(
+          do_QueryInterface(thisController));  // get identity
+      if (thisControllerSup == controllerSup) {
+        mControllers.RemoveElementAt(i);
+        delete controllerData;
+        return NS_OK;
+      }
+    }
+  }
+  return NS_ERROR_FAILURE;  // right thing to return if no controller found?
+}
+
+NS_IMETHODIMP
+nsXULControllers::GetControllerId(nsIController* controller,
+                                  uint32_t* _retval) {
+  NS_ENSURE_ARG_POINTER(_retval);
+
+  uint32_t count = mControllers.Length();
+  for (uint32_t i = 0; i < count; i++) {
+    nsXULControllerData* controllerData = mControllers.ElementAt(i);
+    if (controllerData) {
+      nsCOMPtr<nsIController> thisController;
+      controllerData->GetController(getter_AddRefs(thisController));
+      if (thisController.get() == controller) {
+        *_retval = controllerData->GetControllerID();
+        return NS_OK;
+      }
+    }
+  }
+  return NS_ERROR_FAILURE;  // none found
+}
+
+NS_IMETHODIMP
+nsXULControllers::GetControllerById(uint32_t controllerID,
+                                    nsIController** _retval) {
+  NS_ENSURE_ARG_POINTER(_retval);
+
+  uint32_t count = mControllers.Length();
+  for (uint32_t i = 0; i < count; i++) {
+    nsXULControllerData* controllerData = mControllers.ElementAt(i);
+    if (controllerData && controllerData->GetControllerID() == controllerID) {
+      return controllerData->GetController(_retval);
+    }
+  }
+  return NS_ERROR_FAILURE;  // none found
+}
+
+NS_IMETHODIMP
+nsXULControllers::GetControllerCount(uint32_t* _retval) {
+  NS_ENSURE_ARG_POINTER(_retval);
+  *_retval = mControllers.Length();
+  return NS_OK;
+}
diff --git a/dom/xul/nsXULControllers.h b/dom/xul/nsXULControllers.h
new file mode 100644
index 0000000000..495cf64e71
--- /dev/null
+++ b/dom/xul/nsXULControllers.h
@@ -0,0 +1,51 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+/*
+
+  The XUL "controllers" object.
+
+*/
+
+#ifndef nsXULControllers_h__
+#define nsXULControllers_h__
+
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+#include "nsIControllers.h"
+#include "nsCycleCollectionParticipant.h"
+
+/* non-XPCOM class for holding controllers and their IDs */
+class nsXULControllerData final {
+ public:
+  nsXULControllerData(uint32_t inControllerID, nsIController* inController);
+  ~nsXULControllerData() = default;
+
+  uint32_t GetControllerID() { return mControllerID; }
+
+  nsresult GetController(nsIController** outController);
+
+  uint32_t mControllerID;
+  nsCOMPtr<nsIController> mController;
+};
+
+class nsXULControllers final : public nsIControllers {
+ public:
+  nsXULControllers();
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(nsXULControllers, nsIControllers)
+  NS_DECL_NSICONTROLLERS
+
+ protected:
+  virtual ~nsXULControllers(void);
+
+  void DeleteControllers();
+
+  nsTArray<nsXULControllerData*> mControllers;
+  uint32_t mCurControllerID;
+};
+
+#endif  // nsXULControllers_h__
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
new file mode 100644
index 0000000000..78c29fb315
--- /dev/null
+++ b/dom/xul/nsXULElement.cpp
@@ -0,0 +1,2121 @@
+/* -*- Mode: C++; tab-width: 4; 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 "nsXULElement.h"
+
+#include <new>
+#include <utility>
+#include "AttrArray.h"
+#include "MainThreadUtils.h"
+#include "ReferrerInfo.h"
+#include "Units.h"
+#include "XULButtonElement.h"
+#include "XULFrameElement.h"
+#include "XULMenuElement.h"
+#include "XULMenuBarElement.h"
+#include "XULPopupElement.h"
+#include "XULResizerElement.h"
+#include "XULTextElement.h"
+#include "XULTooltipElement.h"
+#include "XULTreeElement.h"
+#include "js/CompilationAndEvaluation.h"
+#include "js/CompileOptions.h"  // JS::CompileOptions, JS::OwningCompileOptions, , JS::ReadOnlyCompileOptions, JS::ReadOnlyDecodeOptions, JS::DecodeOptions
+#include "js/experimental/CompileScript.h"  // JS::NewFrontendContext, JS::DestroyFrontendContext, JS::SetNativeStackQuota, JS::ThreadStackQuotaForSize, JS::CompileGlobalScriptToStencil, JS::CompilationStorage
+#include "js/experimental/JSStencil.h"      // JS::Stencil, JS::FrontendContext
+#include "js/SourceText.h"
+#include "js/Transcoding.h"
+#include "js/Utility.h"
+#include "jsapi.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/ArrayIterator.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/DeclarationBlock.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/EventListenerManager.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/EventStateManager.h"
+#include "mozilla/FlushType.h"
+#include "mozilla/GlobalKeyListener.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MouseEvents.h"
+#include "mozilla/OwningNonNull.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/ShutdownPhase.h"
+#include "mozilla/StaticAnalysisFunctions.h"
+#include "mozilla/StaticPrefs_javascript.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/TaskController.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/URLExtraData.h"
+#include "mozilla/dom/BindContext.h"
+#include "mozilla/dom/BorrowedAttrInfo.h"
+#include "mozilla/dom/CSSRuleBinding.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/EventTarget.h"
+#include "mozilla/dom/FragmentOrElement.h"
+#include "mozilla/dom/FromParser.h"
+#include "mozilla/dom/MouseEventBinding.h"
+#include "mozilla/dom/MutationEventBinding.h"
+#include "mozilla/dom/NodeInfo.h"
+#include "mozilla/dom/ReferrerPolicyBinding.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/XULBroadcastManager.h"
+#include "mozilla/dom/XULCommandEvent.h"
+#include "mozilla/dom/XULElementBinding.h"
+#include "mozilla/dom/nsCSPUtils.h"
+#include "mozilla/fallible.h"
+#include "nsAtom.h"
+#include "nsAttrValueInlines.h"
+#include "nsCaseTreatment.h"
+#include "nsChangeHint.h"
+#include "nsCOMPtr.h"
+#include "nsCompatibility.h"
+#include "nsContentCreatorFunctions.h"
+#include "nsContentUtils.h"
+#include "nsCycleCollectionNoteChild.h"
+#include "nsCycleCollectionTraversalCallback.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsFocusManager.h"
+#include "nsGkAtoms.h"
+#include "nsIContent.h"
+#include "nsIContentSecurityPolicy.h"
+#include "nsIControllers.h"
+#include "nsID.h"
+#include "nsIDOMEventListener.h"
+#include "nsIDOMXULControlElement.h"
+#include "nsIDOMXULSelectCntrlItemEl.h"
+#include "nsIDocShell.h"
+#include "nsIFocusManager.h"
+#include "nsIFrame.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIRunnable.h"
+#include "nsIScriptContext.h"
+#include "nsISupportsUtils.h"
+#include "nsIURI.h"
+#include "nsIXPConnect.h"
+#include "nsMenuPopupFrame.h"
+#include "nsNodeInfoManager.h"
+#include "nsPIDOMWindow.h"
+#include "nsPIDOMWindowInlines.h"
+#include "nsPresContext.h"
+#include "nsQueryFrame.h"
+#include "nsString.h"
+#include "nsStyledElement.h"
+#include "nsThreadUtils.h"
+#include "nsXULControllers.h"
+#include "nsXULPopupListener.h"
+#include "nsXULPopupManager.h"
+#include "nsXULPrototypeCache.h"
+#include "nsXULTooltipListener.h"
+#include "xpcpublic.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+#ifdef XUL_PROTOTYPE_ATTRIBUTE_METERING
+uint32_t nsXULPrototypeAttribute::gNumElements;
+uint32_t nsXULPrototypeAttribute::gNumAttributes;
+uint32_t nsXULPrototypeAttribute::gNumCacheTests;
+uint32_t nsXULPrototypeAttribute::gNumCacheHits;
+uint32_t nsXULPrototypeAttribute::gNumCacheSets;
+uint32_t nsXULPrototypeAttribute::gNumCacheFills;
+#endif
+
+#define NS_DISPATCH_XUL_COMMAND (1 << 0)
+
+//----------------------------------------------------------------------
+// nsXULElement
+//
+
+nsXULElement::nsXULElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo)
+    : nsStyledElement(std::move(aNodeInfo)) {
+  XUL_PROTOTYPE_ATTRIBUTE_METER(gNumElements);
+}
+
+nsXULElement::~nsXULElement() = default;
+
+/* static */
+nsXULElement* NS_NewBasicXULElement(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo) {
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo(std::move(aNodeInfo));
+  auto* nim = nodeInfo->NodeInfoManager();
+  return new (nim) nsXULElement(nodeInfo.forget());
+}
+
+/* static */
+nsXULElement* nsXULElement::Construct(
+    already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo) {
+  // NOTE: If you add elements here, you probably also want to change
+  // mozilla::dom::binding_detail::HTMLConstructor in BindingUtils.cpp to take
+  // them into account, otherwise you'll start getting "Illegal constructor"
+  // exceptions in chrome code.
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo = aNodeInfo;
+  if (nodeInfo->Equals(nsGkAtoms::resizer)) {
+    return NS_NewXULResizerElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::label) ||
+      nodeInfo->Equals(nsGkAtoms::description)) {
+    auto* nim = nodeInfo->NodeInfoManager();
+    return new (nim) XULTextElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::menupopup) ||
+      nodeInfo->Equals(nsGkAtoms::panel)) {
+    return NS_NewXULPopupElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::tooltip)) {
+    return NS_NewXULTooltipElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::iframe) ||
+      nodeInfo->Equals(nsGkAtoms::browser) ||
+      nodeInfo->Equals(nsGkAtoms::editor)) {
+    auto* nim = nodeInfo->NodeInfoManager();
+    return new (nim) XULFrameElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::menubar)) {
+    auto* nim = nodeInfo->NodeInfoManager();
+    return new (nim) XULMenuBarElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::menu) ||
+      nodeInfo->Equals(nsGkAtoms::menulist)) {
+    auto* nim = nodeInfo->NodeInfoManager();
+    return new (nim) XULMenuElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::tree)) {
+    auto* nim = nodeInfo->NodeInfoManager();
+    return new (nim) XULTreeElement(nodeInfo.forget());
+  }
+
+  if (nodeInfo->Equals(nsGkAtoms::checkbox) ||
+      nodeInfo->Equals(nsGkAtoms::radio) ||
+      nodeInfo->Equals(nsGkAtoms::thumb) ||
+      nodeInfo->Equals(nsGkAtoms::button) ||
+      nodeInfo->Equals(nsGkAtoms::menuitem) ||
+      nodeInfo->Equals(nsGkAtoms::toolbarbutton) ||
+      nodeInfo->Equals(nsGkAtoms::toolbarpaletteitem) ||
+      nodeInfo->Equals(nsGkAtoms::scrollbarbutton)) {
+    auto* nim = nodeInfo->NodeInfoManager();
+    return new (nim) XULButtonElement(nodeInfo.forget());
+  }
+
+  return NS_NewBasicXULElement(nodeInfo.forget());
+}
+
+/* static */
+already_AddRefed<nsXULElement> nsXULElement::CreateFromPrototype(
+    nsXULPrototypeElement* aPrototype, mozilla::dom::NodeInfo* aNodeInfo,
+    bool aIsScriptable, bool aIsRoot) {
+  RefPtr<mozilla::dom::NodeInfo> ni = aNodeInfo;
+  nsCOMPtr<Element> baseElement;
+  NS_NewXULElement(getter_AddRefs(baseElement), ni.forget(),
+                   dom::FROM_PARSER_NETWORK, aPrototype->mIsAtom);
+
+  if (baseElement) {
+    nsXULElement* element = FromNode(baseElement);
+
+    if (aPrototype->mHasIdAttribute) {
+      element->SetHasID();
+    }
+    if (aPrototype->mHasClassAttribute) {
+      element->SetMayHaveClass();
+    }
+    if (aPrototype->mHasStyleAttribute) {
+      element->SetMayHaveStyle();
+    }
+
+    element->MakeHeavyweight(aPrototype);
+    if (aIsScriptable) {
+      // Check each attribute on the prototype to see if we need to do
+      // any additional processing and hookup that would otherwise be
+      // done 'automagically' by SetAttr().
+      for (const auto& attribute : aPrototype->mAttributes) {
+        element->AddListenerForAttributeIfNeeded(attribute.mName);
+      }
+    }
+
+    return baseElement.forget().downcast<nsXULElement>();
+  }
+
+  return nullptr;
+}
+
+nsresult nsXULElement::CreateFromPrototype(nsXULPrototypeElement* aPrototype,
+                                           Document* aDocument,
+                                           bool aIsScriptable, bool aIsRoot,
+                                           Element** aResult) {
+  // Create an nsXULElement from a prototype
+  MOZ_ASSERT(aPrototype != nullptr, "null ptr");
+  if (!aPrototype) return NS_ERROR_NULL_POINTER;
+
+  MOZ_ASSERT(aResult != nullptr, "null ptr");
+  if (!aResult) return NS_ERROR_NULL_POINTER;
+
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo;
+  if (aDocument) {
+    mozilla::dom::NodeInfo* ni = aPrototype->mNodeInfo;
+    nodeInfo = aDocument->NodeInfoManager()->GetNodeInfo(
+        ni->NameAtom(), ni->GetPrefixAtom(), ni->NamespaceID(), ELEMENT_NODE);
+  } else {
+    nodeInfo = aPrototype->mNodeInfo;
+  }
+
+  RefPtr<nsXULElement> element =
+      CreateFromPrototype(aPrototype, nodeInfo, aIsScriptable, aIsRoot);
+  element.forget(aResult);
+
+  return NS_OK;
+}
+
+nsresult NS_NewXULElement(Element** aResult,
+                          already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
+                          FromParser aFromParser, nsAtom* aIsAtom,
+                          mozilla::dom::CustomElementDefinition* aDefinition) {
+  RefPtr<mozilla::dom::NodeInfo> nodeInfo = aNodeInfo;
+
+  MOZ_ASSERT(nodeInfo, "need nodeinfo for non-proto Create");
+
+  NS_ASSERTION(
+      nodeInfo->NamespaceEquals(kNameSpaceID_XUL),
+      "Trying to create XUL elements that don't have the XUL namespace");
+
+  Document* doc = nodeInfo->GetDocument();
+  if (doc && !doc->AllowXULXBL()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  return nsContentUtils::NewXULOrHTMLElement(aResult, nodeInfo, aFromParser,
+                                             aIsAtom, aDefinition);
+}
+
+void NS_TrustedNewXULElement(
+    Element** aResult, already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo) {
+  RefPtr<mozilla::dom::NodeInfo> ni = aNodeInfo;
+  MOZ_ASSERT(ni, "need nodeinfo for non-proto Create");
+
+  // Create an nsXULElement with the specified namespace and tag.
+  NS_ADDREF(*aResult = nsXULElement::Construct(ni.forget()));
+}
+
+//----------------------------------------------------------------------
+// nsISupports interface
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(nsXULElement, nsStyledElement)
+
+NS_IMPL_ADDREF_INHERITED(nsXULElement, nsStyledElement)
+NS_IMPL_RELEASE_INHERITED(nsXULElement, nsStyledElement)
+
+NS_INTERFACE_TABLE_HEAD_CYCLE_COLLECTION_INHERITED(nsXULElement)
+  NS_ELEMENT_INTERFACE_TABLE_TO_MAP_SEGUE
+NS_INTERFACE_MAP_END_INHERITING(nsStyledElement)
+
+//----------------------------------------------------------------------
+// nsINode interface
+
+nsresult nsXULElement::Clone(mozilla::dom::NodeInfo* aNodeInfo,
+                             nsINode** aResult) const {
+  *aResult = nullptr;
+
+  RefPtr<mozilla::dom::NodeInfo> ni = aNodeInfo;
+  RefPtr<nsXULElement> element = Construct(ni.forget());
+
+  nsresult rv = const_cast<nsXULElement*>(this)->CopyInnerTo(
+      element, ReparseAttributes::No);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Note that we're _not_ copying mControllers.
+
+  element.forget(aResult);
+  return rv;
+}
+
+//----------------------------------------------------------------------
+
+EventListenerManager* nsXULElement::GetEventListenerManagerForAttr(
+    nsAtom* aAttrName, bool* aDefer) {
+  // XXXbz sXBL/XBL2 issue: should we instead use GetComposedDoc()
+  // here, override BindToTree for those classes and munge event
+  // listeners there?
+  Document* doc = OwnerDoc();
+
+  nsPIDOMWindowInner* window;
+  Element* root = doc->GetRootElement();
+  if ((!root || root == this) && (window = doc->GetInnerWindow())) {
+    nsCOMPtr<EventTarget> piTarget = do_QueryInterface(window);
+
+    *aDefer = false;
+    return piTarget->GetOrCreateListenerManager();
+  }
+
+  return nsStyledElement::GetEventListenerManagerForAttr(aAttrName, aDefer);
+}
+
+// returns true if the element is not a list
+static bool IsNonList(mozilla::dom::NodeInfo* aNodeInfo) {
+  return !aNodeInfo->Equals(nsGkAtoms::tree) &&
+         !aNodeInfo->Equals(nsGkAtoms::richlistbox);
+}
+
+nsXULElement::XULFocusability nsXULElement::GetXULFocusability(
+    bool aWithMouse) {
+#ifdef XP_MACOSX
+  // On Mac, mouse interactions only focus the element if it's a list,
+  // or if it's a remote target, since the remote target must handle
+  // the focus.
+  if (aWithMouse && IsNonList(mNodeInfo) &&
+      !EventStateManager::IsTopLevelRemoteTarget(this)) {
+    return XULFocusability::NeverFocusable();
+  }
+#endif
+
+  XULFocusability result;
+  nsCOMPtr<nsIDOMXULControlElement> xulControl = AsXULControl();
+  if (xulControl) {
+    // A disabled element cannot be focused and is not part of the tab order
+    bool disabled;
+    xulControl->GetDisabled(&disabled);
+    if (disabled) {
+      return XULFocusability::NeverFocusable();
+    }
+    result.mDefaultFocusable = true;
+  }
+  if (Maybe<int32_t> attrVal = GetTabIndexAttrValue()) {
+    // The tabindex attribute was specified, so the element becomes
+    // focusable.
+    result.mDefaultFocusable = true;
+    result.mForcedFocusable.emplace(true);
+    result.mForcedTabIndexIfFocusable.emplace(attrVal.value());
+  }
+  if (xulControl && sTabFocusModelAppliesToXUL &&
+      !(sTabFocusModel & eTabFocus_formElementsMask) && IsNonList(mNodeInfo)) {
+    // By default, the tab focus model doesn't apply to xul element on any
+    // system but OS X. on OS X we're following it for UI elements (XUL) as
+    // sTabFocusModel is based on "Full Keyboard Access" system setting (see
+    // mac/nsILookAndFeel). both textboxes and list elements (i.e. trees and
+    // list) should always be focusable (textboxes are handled as html:input)
+    // For compatibility, we only do this for controls, otherwise elements
+    // like <browser> cannot take this focus.
+    result.mForcedTabIndexIfFocusable = Some(-1);
+  }
+  return result;
+}
+
+// XUL elements are not focusable unless explicitly opted-into it with
+// -moz-user-focus: normal, or the tabindex attribute.
+Focusable nsXULElement::IsFocusableWithoutStyle(bool aWithMouse) {
+  const auto focusability = GetXULFocusability(aWithMouse);
+  const bool focusable = focusability.mDefaultFocusable;
+  return {focusable,
+          focusable ? focusability.mForcedTabIndexIfFocusable.valueOr(-1) : -1};
+}
+
+bool nsXULElement::HasMenu() {
+  if (auto* button = XULButtonElement::FromNode(this)) {
+    return button->IsMenu();
+  }
+  return false;
+}
+
+void nsXULElement::OpenMenu(bool aOpenFlag) {
+  // Flush frames first. It's not clear why this is needed, see bug 1704670.
+  if (Document* doc = GetComposedDoc()) {
+    doc->FlushPendingNotifications(FlushType::Frames);
+  }
+
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (!pm) {
+    return;
+  }
+
+  if (aOpenFlag) {
+    // Nothing will happen if this element isn't a menu.
+    pm->ShowMenu(this, false);
+  } else {
+    // Nothing will happen if this element isn't a menu.
+    pm->HideMenu(this);
+  }
+}
+
+Result<bool, nsresult> nsXULElement::PerformAccesskey(bool aKeyCausesActivation,
+                                                      bool aIsTrustedEvent) {
+  if (IsXULElement(nsGkAtoms::label)) {
+    nsAutoString control;
+    GetAttr(nsGkAtoms::control, control);
+    if (control.IsEmpty()) {
+      return Err(NS_ERROR_UNEXPECTED);
+    }
+
+    // XXXsmaug Should we use ShadowRoot::GetElementById in case
+    //          element is in Shadow DOM?
+    RefPtr<Document> document = GetUncomposedDoc();
+    if (!document) {
+      return Err(NS_ERROR_UNEXPECTED);
+    }
+
+    RefPtr<Element> element = document->GetElementById(control);
+    if (!element) {
+      return Err(NS_ERROR_UNEXPECTED);
+    }
+
+    // XXXedgar, This is mainly for HTMLElement which doesn't do visible
+    // check in PerformAccesskey. We probably should always do visible
+    // check on HTMLElement even if the PerformAccesskey is not redirected from
+    // label XULelement per spec.
+    nsIFrame* frame = element->GetPrimaryFrame();
+    if (!frame || !frame->IsVisibleConsideringAncestors()) {
+      return Err(NS_ERROR_UNEXPECTED);
+    }
+
+    return element->PerformAccesskey(aKeyCausesActivation, aIsTrustedEvent);
+  }
+
+  nsIFrame* frame = GetPrimaryFrame();
+  if (!frame || !frame->IsVisibleConsideringAncestors()) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  bool focused = false;
+  // Define behavior for each type of XUL element.
+  if (!IsXULElement(nsGkAtoms::toolbarbutton)) {
+    if (RefPtr<nsFocusManager> fm = nsFocusManager::GetFocusManager()) {
+      RefPtr<Element> elementToFocus = this;
+      // for radio buttons, focus the radiogroup instead
+      if (IsXULElement(nsGkAtoms::radio)) {
+        if (nsCOMPtr<nsIDOMXULSelectControlItemElement> controlItem =
+                AsXULSelectControlItem()) {
+          bool disabled;
+          controlItem->GetDisabled(&disabled);
+          if (!disabled) {
+            controlItem->GetControl(getter_AddRefs(elementToFocus));
+          }
+        }
+      }
+
+      if (elementToFocus) {
+        fm->SetFocus(elementToFocus, nsIFocusManager::FLAG_BYKEY);
+
+        // Return true if the element became focused.
+        nsPIDOMWindowOuter* window = OwnerDoc()->GetWindow();
+        focused = (window && window->GetFocusedElement() == elementToFocus);
+      }
+    }
+  }
+
+  if (aKeyCausesActivation && !IsXULElement(nsGkAtoms::menulist)) {
+    ClickWithInputSource(MouseEvent_Binding::MOZ_SOURCE_KEYBOARD,
+                         aIsTrustedEvent);
+    return focused;
+  }
+
+  // If the accesskey won't cause the activation and the focus isn't changed,
+  // either. Return error so EventStateManager would try to find next element
+  // to handle the accesskey.
+  return focused ? Result<bool, nsresult>{focused} : Err(NS_ERROR_ABORT);
+}
+
+//----------------------------------------------------------------------
+
+void nsXULElement::AddListenerForAttributeIfNeeded(nsAtom* aLocalName) {
+  // If appropriate, add a popup listener and/or compile the event
+  // handler. Called when we change the element's document, create a
+  // new element, change an attribute's value, etc.
+  // Eventlistenener-attributes are always in the null namespace.
+  if (aLocalName == nsGkAtoms::menu || aLocalName == nsGkAtoms::contextmenu ||
+      // XXXdwh popup and context are deprecated
+      aLocalName == nsGkAtoms::popup || aLocalName == nsGkAtoms::context) {
+    AddPopupListener(aLocalName);
+  }
+  if (nsContentUtils::IsEventAttributeName(aLocalName, EventNameType_XUL)) {
+    nsAutoString value;
+    GetAttr(aLocalName, value);
+    SetEventHandler(aLocalName, value, true);
+  }
+}
+
+void nsXULElement::AddListenerForAttributeIfNeeded(const nsAttrName& aName) {
+  if (aName.IsAtom()) {
+    AddListenerForAttributeIfNeeded(aName.Atom());
+  }
+}
+
+class XULInContentErrorReporter : public Runnable {
+ public:
+  explicit XULInContentErrorReporter(Document& aDocument)
+      : mozilla::Runnable("XULInContentErrorReporter"), mDocument(aDocument) {}
+
+  NS_IMETHOD Run() override {
+    mDocument->WarnOnceAbout(DeprecatedOperations::eImportXULIntoContent,
+                             false);
+    return NS_OK;
+  }
+
+ private:
+  OwningNonNull<Document> mDocument;
+};
+
+static bool NeedTooltipSupport(const nsXULElement& aXULElement) {
+  if (aXULElement.NodeInfo()->Equals(nsGkAtoms::treechildren)) {
+    // treechildren always get tooltip support, since cropped tree cells show
+    // their full text in a tooltip.
+    return true;
+  }
+
+  return aXULElement.GetBoolAttr(nsGkAtoms::tooltip) ||
+         aXULElement.GetBoolAttr(nsGkAtoms::tooltiptext);
+}
+
+nsresult nsXULElement::BindToTree(BindContext& aContext, nsINode& aParent) {
+  nsresult rv = nsStyledElement::BindToTree(aContext, aParent);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (!IsInComposedDoc()) {
+    return rv;
+  }
+
+  Document& doc = aContext.OwnerDoc();
+  if (!IsInNativeAnonymousSubtree() && !doc.AllowXULXBL() &&
+      !doc.HasWarnedAbout(DeprecatedOperations::eImportXULIntoContent)) {
+    nsContentUtils::AddScriptRunner(new XULInContentErrorReporter(doc));
+  }
+
+#ifdef DEBUG
+  if (!doc.AllowXULXBL() && !doc.IsUnstyledDocument()) {
+    // To save CPU cycles and memory, we don't load xul.css for other elements
+    // except scrollbars.
+    //
+    // This assertion makes sure no other XUL element is used in a non-XUL
+    // document.
+    nsAtom* tag = NodeInfo()->NameAtom();
+    MOZ_ASSERT(tag == nsGkAtoms::scrollbar ||
+                   tag == nsGkAtoms::scrollbarbutton ||
+                   tag == nsGkAtoms::scrollcorner || tag == nsGkAtoms::slider ||
+                   tag == nsGkAtoms::thumb || tag == nsGkAtoms::resizer,
+               "Unexpected XUL element in non-XUL doc");
+  }
+#endif
+
+  // Within Bug 1492063 and its dependencies we started to apply a
+  // CSP to system privileged about pages. Since some about: pages
+  // are implemented in *.xul files we added this workaround to
+  // apply a CSP to them. To do so, we check the introduced custom
+  // attribute 'csp' on the root element.
+  if (doc.GetRootElement() == this) {
+    nsAutoString cspPolicyStr;
+    GetAttr(nsGkAtoms::csp, cspPolicyStr);
+
+#ifdef DEBUG
+    {
+      nsCOMPtr<nsIContentSecurityPolicy> docCSP = doc.GetCsp();
+      uint32_t policyCount = 0;
+      if (docCSP) {
+        docCSP->GetPolicyCount(&policyCount);
+      }
+      MOZ_ASSERT(policyCount == 0, "how come we already have a policy?");
+    }
+#endif
+
+    CSP_ApplyMetaCSPToDoc(doc, cspPolicyStr);
+  }
+
+  if (NodeInfo()->Equals(nsGkAtoms::keyset, kNameSpaceID_XUL)) {
+    // Create our XUL key listener and hook it up.
+    XULKeySetGlobalKeyListener::AttachKeyHandler(this);
+  }
+
+  RegUnRegAccessKey(true);
+
+  if (NeedTooltipSupport(*this)) {
+    AddTooltipSupport();
+  }
+
+  if (XULBroadcastManager::MayNeedListener(*this)) {
+    if (!doc.HasXULBroadcastManager()) {
+      doc.InitializeXULBroadcastManager();
+    }
+    XULBroadcastManager* broadcastManager = doc.GetXULBroadcastManager();
+    broadcastManager->AddListener(this);
+  }
+  return rv;
+}
+
+void nsXULElement::UnbindFromTree(bool aNullParent) {
+  if (NodeInfo()->Equals(nsGkAtoms::keyset, kNameSpaceID_XUL)) {
+    XULKeySetGlobalKeyListener::DetachKeyHandler(this);
+  }
+
+  RegUnRegAccessKey(false);
+
+  if (NeedTooltipSupport(*this)) {
+    RemoveTooltipSupport();
+  }
+
+  Document* doc = GetComposedDoc();
+  if (doc && doc->HasXULBroadcastManager() &&
+      XULBroadcastManager::MayNeedListener(*this)) {
+    RefPtr<XULBroadcastManager> broadcastManager =
+        doc->GetXULBroadcastManager();
+    broadcastManager->RemoveListener(this);
+  }
+
+  // mControllers can own objects that are implemented
+  // in JavaScript (such as some implementations of
+  // nsIControllers.  These objects prevent their global
+  // object's script object from being garbage collected,
+  // which means JS continues to hold an owning reference
+  // to the nsGlobalWindow, which owns the document,
+  // which owns this content.  That's a cycle, so we break
+  // it here.  (It might be better to break this by releasing
+  // mDocument in nsGlobalWindow::SetDocShell, but I'm not
+  // sure whether that would fix all possible cycles through
+  // mControllers.)
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
+  if (slots) {
+    slots->mControllers = nullptr;
+  }
+
+  nsStyledElement::UnbindFromTree(aNullParent);
+}
+
+void nsXULElement::DoneAddingChildren(bool aHaveNotified) {
+  if (IsXULElement(nsGkAtoms::linkset)) {
+    Document* doc = GetComposedDoc();
+    if (doc) {
+      doc->OnL10nResourceContainerParsed();
+    }
+  }
+}
+
+void nsXULElement::RegUnRegAccessKey(bool aDoReg) {
+  // Don't try to register for unsupported elements
+  if (!SupportsAccessKey()) {
+    return;
+  }
+
+  nsStyledElement::RegUnRegAccessKey(aDoReg);
+}
+
+bool nsXULElement::SupportsAccessKey() const {
+  if (NodeInfo()->Equals(nsGkAtoms::label) && HasAttr(nsGkAtoms::control)) {
+    return true;
+  }
+
+  // XXX(ntim): check if description[value] or description[accesskey] are
+  // actually used, remove `value` from {Before/After}SetAttr if not the case
+  if (NodeInfo()->Equals(nsGkAtoms::description) && HasAttr(nsGkAtoms::value) &&
+      HasAttr(nsGkAtoms::control)) {
+    return true;
+  }
+
+  return IsAnyOfXULElements(nsGkAtoms::button, nsGkAtoms::toolbarbutton,
+                            nsGkAtoms::checkbox, nsGkAtoms::tab,
+                            nsGkAtoms::radio);
+}
+
+void nsXULElement::BeforeSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                                 const nsAttrValue* aValue, bool aNotify) {
+  if (aNamespaceID == kNameSpaceID_None) {
+    if (aName == nsGkAtoms::accesskey || aName == nsGkAtoms::control ||
+        aName == nsGkAtoms::value) {
+      RegUnRegAccessKey(false);
+    } else if ((aName == nsGkAtoms::command || aName == nsGkAtoms::observes) &&
+               IsInUncomposedDoc()) {
+      //         XXX sXBL/XBL2 issue! Owner or current document?
+      // XXX Why does this not also remove broadcast listeners if the
+      // "element" attribute was changed on an <observer>?
+      nsAutoString oldValue;
+      GetAttr(nsGkAtoms::observes, oldValue);
+      if (oldValue.IsEmpty()) {
+        GetAttr(nsGkAtoms::command, oldValue);
+      }
+      Document* doc = GetUncomposedDoc();
+      if (!oldValue.IsEmpty() && doc->HasXULBroadcastManager()) {
+        RefPtr<XULBroadcastManager> broadcastManager =
+            doc->GetXULBroadcastManager();
+        broadcastManager->RemoveListener(this);
+      }
+#ifdef DEBUG
+    } else if (aName == nsGkAtoms::usercontextid) {
+      const nsAttrValue* oldValue = GetParsedAttr(aName);
+      if (oldValue && (!aValue || !aValue->Equals(*oldValue))) {
+        MOZ_ASSERT(false,
+                   "Changing usercontextid doesn't really work properly.");
+      }
+#endif
+    }
+  }
+
+  return nsStyledElement::BeforeSetAttr(aNamespaceID, aName, aValue, aNotify);
+}
+
+void nsXULElement::AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                                const nsAttrValue* aValue,
+                                const nsAttrValue* aOldValue,
+                                nsIPrincipal* aSubjectPrincipal, bool aNotify) {
+  if (aNamespaceID == kNameSpaceID_None) {
+    if (aValue) {
+      AddListenerForAttributeIfNeeded(aName);
+    }
+
+    if (aName == nsGkAtoms::accesskey || aName == nsGkAtoms::control ||
+        aName == nsGkAtoms::value) {
+      RegUnRegAccessKey(true);
+    } else if (aName == nsGkAtoms::tooltip || aName == nsGkAtoms::tooltiptext) {
+      if (!!aValue != !!aOldValue && IsInComposedDoc() &&
+          !NodeInfo()->Equals(nsGkAtoms::treechildren)) {
+        if (aValue) {
+          AddTooltipSupport();
+        } else {
+          RemoveTooltipSupport();
+        }
+      }
+    }
+    Document* doc = GetComposedDoc();
+    if (doc && doc->HasXULBroadcastManager()) {
+      RefPtr<XULBroadcastManager> broadcastManager =
+          doc->GetXULBroadcastManager();
+      broadcastManager->AttributeChanged(this, aNamespaceID, aName);
+    }
+    if (doc && XULBroadcastManager::MayNeedListener(*this)) {
+      if (!doc->HasXULBroadcastManager()) {
+        doc->InitializeXULBroadcastManager();
+      }
+      XULBroadcastManager* broadcastManager = doc->GetXULBroadcastManager();
+      broadcastManager->AddListener(this);
+    }
+
+    // XXX need to check if they're changing an event handler: if
+    // so, then we need to unhook the old one.  Or something.
+  }
+
+  return nsStyledElement::AfterSetAttr(aNamespaceID, aName, aValue, aOldValue,
+                                       aSubjectPrincipal, aNotify);
+}
+
+void nsXULElement::AddTooltipSupport() {
+  nsXULTooltipListener* listener = nsXULTooltipListener::GetInstance();
+  if (!listener) {
+    return;
+  }
+
+  listener->AddTooltipSupport(this);
+}
+
+void nsXULElement::RemoveTooltipSupport() {
+  nsXULTooltipListener* listener = nsXULTooltipListener::GetInstance();
+  if (!listener) {
+    return;
+  }
+
+  listener->RemoveTooltipSupport(this);
+}
+
+bool nsXULElement::ParseAttribute(int32_t aNamespaceID, nsAtom* aAttribute,
+                                  const nsAString& aValue,
+                                  nsIPrincipal* aMaybeScriptedPrincipal,
+                                  nsAttrValue& aResult) {
+  if (aNamespaceID == kNameSpaceID_None && aAttribute == nsGkAtoms::tabindex) {
+    return aResult.ParseIntValue(aValue);
+  }
+
+  // Parse into a nsAttrValue
+  if (!nsStyledElement::ParseAttribute(aNamespaceID, aAttribute, aValue,
+                                       aMaybeScriptedPrincipal, aResult)) {
+    // Fall back to parsing as atom for short values
+    aResult.ParseStringOrAtom(aValue);
+  }
+
+  return true;
+}
+
+void nsXULElement::DestroyContent() {
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
+  if (slots) {
+    slots->mControllers = nullptr;
+  }
+
+  nsStyledElement::DestroyContent();
+}
+
+#ifdef MOZ_DOM_LIST
+void nsXULElement::List(FILE* out, int32_t aIndent) const {
+  nsCString prefix("XUL");
+  if (HasSlots()) {
+    prefix.Append('*');
+  }
+  prefix.Append(' ');
+
+  nsStyledElement::List(out, aIndent, prefix);
+}
+#endif
+
+bool nsXULElement::IsEventStoppedFromAnonymousScrollbar(EventMessage aMessage) {
+  return (IsRootOfNativeAnonymousSubtree() &&
+          IsAnyOfXULElements(nsGkAtoms::scrollbar, nsGkAtoms::scrollcorner) &&
+          (aMessage == eMouseClick || aMessage == eMouseDoubleClick ||
+           aMessage == eXULCommand || aMessage == eContextMenu ||
+           aMessage == eDragStart || aMessage == eMouseAuxClick));
+}
+
+nsresult nsXULElement::DispatchXULCommand(const EventChainVisitor& aVisitor,
+                                          nsAutoString& aCommand) {
+  // XXX sXBL/XBL2 issue! Owner or current document?
+  nsCOMPtr<Document> doc = GetUncomposedDoc();
+  NS_ENSURE_STATE(doc);
+  RefPtr<Element> commandElt = doc->GetElementById(aCommand);
+  if (commandElt) {
+    // Create a new command event to dispatch to the element
+    // pointed to by the command attribute. The new event's
+    // sourceEvent will be the original command event that we're
+    // handling.
+    RefPtr<Event> event = aVisitor.mDOMEvent;
+    uint16_t inputSource = MouseEvent_Binding::MOZ_SOURCE_UNKNOWN;
+    int16_t button = 0;
+    while (event) {
+      NS_ENSURE_STATE(event->GetOriginalTarget() != commandElt);
+      RefPtr<XULCommandEvent> commandEvent = event->AsXULCommandEvent();
+      if (commandEvent) {
+        event = commandEvent->GetSourceEvent();
+        inputSource = commandEvent->InputSource();
+        button = commandEvent->Button();
+      } else {
+        event = nullptr;
+      }
+    }
+    WidgetInputEvent* orig = aVisitor.mEvent->AsInputEvent();
+    nsContentUtils::DispatchXULCommand(
+        commandElt, orig->IsTrusted(), MOZ_KnownLive(aVisitor.mDOMEvent),
+        nullptr, orig->IsControl(), orig->IsAlt(), orig->IsShift(),
+        orig->IsMeta(), inputSource, button);
+  } else {
+    NS_WARNING("A XUL element is attached to a command that doesn't exist!\n");
+  }
+  return NS_OK;
+}
+
+void nsXULElement::GetEventTargetParent(EventChainPreVisitor& aVisitor) {
+  aVisitor.mForceContentDispatch = true;  // FIXME! Bug 329119
+  if (IsEventStoppedFromAnonymousScrollbar(aVisitor.mEvent->mMessage)) {
+    // Don't propagate these events from native anonymous scrollbar.
+    aVisitor.mCanHandle = true;
+    aVisitor.SetParentTarget(nullptr, false);
+    return;
+  }
+  if (aVisitor.mEvent->mMessage == eXULCommand &&
+      aVisitor.mEvent->mClass == eInputEventClass &&
+      aVisitor.mEvent->mOriginalTarget == static_cast<nsIContent*>(this) &&
+      !IsXULElement(nsGkAtoms::command)) {
+    // Check that we really have an xul command event. That will be handled
+    // in a special way.
+    // See if we have a command elt.  If so, we execute on the command
+    // instead of on our content element.
+    if (aVisitor.mDOMEvent && aVisitor.mDOMEvent->AsXULCommandEvent() &&
+        HasNonEmptyAttr(nsGkAtoms::command)) {
+      // Stop building the event target chain for the original event.
+      // We don't want it to propagate to any DOM nodes.
+      aVisitor.mCanHandle = false;
+      aVisitor.mAutomaticChromeDispatch = false;
+      // Dispatch XUL command in PreHandleEvent to prevent it breaks event
+      // target chain creation
+      aVisitor.mWantsPreHandleEvent = true;
+      aVisitor.mItemFlags |= NS_DISPATCH_XUL_COMMAND;
+      return;
+    }
+  }
+
+  nsStyledElement::GetEventTargetParent(aVisitor);
+}
+
+nsresult nsXULElement::PreHandleEvent(EventChainVisitor& aVisitor) {
+  if (aVisitor.mItemFlags & NS_DISPATCH_XUL_COMMAND) {
+    nsAutoString command;
+    GetAttr(nsGkAtoms::command, command);
+    MOZ_ASSERT(!command.IsEmpty());
+    return DispatchXULCommand(aVisitor, command);
+  }
+  return nsStyledElement::PreHandleEvent(aVisitor);
+}
+
+//----------------------------------------------------------------------
+// Implementation methods
+
+NS_IMETHODIMP_(bool)
+nsXULElement::IsAttributeMapped(const nsAtom* aAttribute) const {
+  return false;
+}
+
+nsIControllers* nsXULElement::GetControllers(ErrorResult& rv) {
+  if (!Controllers()) {
+    nsExtendedDOMSlots* slots = ExtendedDOMSlots();
+
+    slots->mControllers = new nsXULControllers();
+  }
+
+  return Controllers();
+}
+
+void nsXULElement::Click(CallerType aCallerType) {
+  ClickWithInputSource(MouseEvent_Binding::MOZ_SOURCE_UNKNOWN,
+                       aCallerType == CallerType::System);
+}
+
+void nsXULElement::ClickWithInputSource(uint16_t aInputSource,
+                                        bool aIsTrustedEvent) {
+  if (BoolAttrIsTrue(nsGkAtoms::disabled)) return;
+
+  nsCOMPtr<Document> doc = GetComposedDoc();  // Strong just in case
+  if (doc) {
+    RefPtr<nsPresContext> context = doc->GetPresContext();
+    if (context) {
+      // strong ref to PresContext so events don't destroy it
+
+      WidgetMouseEvent eventDown(aIsTrustedEvent, eMouseDown, nullptr,
+                                 WidgetMouseEvent::eReal);
+      WidgetMouseEvent eventUp(aIsTrustedEvent, eMouseUp, nullptr,
+                               WidgetMouseEvent::eReal);
+      // This helps to avoid commands being dispatched from
+      // XULButtonElement::PostHandleEventForMenu.
+      eventUp.mFlags.mMultipleActionsPrevented = true;
+      WidgetMouseEvent eventClick(aIsTrustedEvent, eMouseClick, nullptr,
+                                  WidgetMouseEvent::eReal);
+      eventDown.mInputSource = eventUp.mInputSource = eventClick.mInputSource =
+          aInputSource;
+
+      // send mouse down
+      nsEventStatus status = nsEventStatus_eIgnore;
+      EventDispatcher::Dispatch(this, context, &eventDown, nullptr, &status);
+
+      // send mouse up
+      status = nsEventStatus_eIgnore;  // reset status
+      EventDispatcher::Dispatch(this, context, &eventUp, nullptr, &status);
+
+      // send mouse click
+      status = nsEventStatus_eIgnore;  // reset status
+      EventDispatcher::Dispatch(this, context, &eventClick, nullptr, &status);
+
+      // If the click has been prevented, lets skip the command call
+      // this is how a physical click works
+      if (status == nsEventStatus_eConsumeNoDefault) {
+        return;
+      }
+    }
+  }
+
+  // oncommand is fired when an element is clicked...
+  DoCommand();
+}
+
+void nsXULElement::DoCommand() {
+  nsCOMPtr<Document> doc = GetComposedDoc();  // strong just in case
+  if (doc) {
+    RefPtr<nsXULElement> self = this;
+    nsContentUtils::DispatchXULCommand(self, true);
+  }
+}
+
+nsresult nsXULElement::AddPopupListener(nsAtom* aName) {
+  // Add a popup listener to the element
+  bool isContext =
+      (aName == nsGkAtoms::context || aName == nsGkAtoms::contextmenu);
+  uint32_t listenerFlag = isContext ? XUL_ELEMENT_HAS_CONTENTMENU_LISTENER
+                                    : XUL_ELEMENT_HAS_POPUP_LISTENER;
+
+  if (HasFlag(listenerFlag)) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIDOMEventListener> listener =
+      new nsXULPopupListener(this, isContext);
+
+  // Add the popup as a listener on this element.
+  EventListenerManager* manager = GetOrCreateListenerManager();
+  SetFlags(listenerFlag);
+
+  if (isContext) {
+    manager->AddEventListenerByType(listener, u"contextmenu"_ns,
+                                    TrustedEventsAtSystemGroupBubble());
+  } else {
+    manager->AddEventListenerByType(listener, u"mousedown"_ns,
+                                    TrustedEventsAtSystemGroupBubble());
+  }
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------
+
+nsresult nsXULElement::MakeHeavyweight(nsXULPrototypeElement* aPrototype) {
+  if (!aPrototype) {
+    return NS_OK;
+  }
+
+  size_t i;
+  nsresult rv;
+  for (i = 0; i < aPrototype->mAttributes.Length(); ++i) {
+    nsXULPrototypeAttribute* protoattr = &aPrototype->mAttributes[i];
+    nsAttrValue attrValue;
+
+    // Style rules need to be cloned.
+    if (protoattr->mValue.Type() == nsAttrValue::eCSSDeclaration) {
+      DeclarationBlock* decl = protoattr->mValue.GetCSSDeclarationValue();
+      RefPtr<DeclarationBlock> declClone = decl->Clone();
+
+      nsString stringValue;
+      protoattr->mValue.ToString(stringValue);
+
+      attrValue.SetTo(declClone.forget(), &stringValue);
+    } else {
+      attrValue.SetTo(protoattr->mValue);
+    }
+
+    bool oldValueSet;
+    // XXX we might wanna have a SetAndTakeAttr that takes an nsAttrName
+    if (protoattr->mName.IsAtom()) {
+      rv = mAttrs.SetAndSwapAttr(protoattr->mName.Atom(), attrValue,
+                                 &oldValueSet);
+    } else {
+      rv = mAttrs.SetAndSwapAttr(protoattr->mName.NodeInfo(), attrValue,
+                                 &oldValueSet);
+    }
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+  return NS_OK;
+}
+
+bool nsXULElement::BoolAttrIsTrue(nsAtom* aName) const {
+  const nsAttrValue* attr = GetAttrInfo(kNameSpaceID_None, aName).mValue;
+
+  return attr && attr->Type() == nsAttrValue::eAtom &&
+         attr->GetAtomValue() == nsGkAtoms::_true;
+}
+
+bool nsXULElement::IsEventAttributeNameInternal(nsAtom* aName) {
+  return nsContentUtils::IsEventAttributeName(aName, EventNameType_XUL);
+}
+
+JSObject* nsXULElement::WrapNode(JSContext* aCx,
+                                 JS::Handle<JSObject*> aGivenProto) {
+  return dom::XULElement_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+bool nsXULElement::IsInteractiveHTMLContent() const {
+  return IsXULElement(nsGkAtoms::menupopup) ||
+         Element::IsInteractiveHTMLContent();
+}
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsXULPrototypeNode)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsXULPrototypeNode)
+  if (tmp->mType == nsXULPrototypeNode::eType_Element) {
+    static_cast<nsXULPrototypeElement*>(tmp)->Unlink();
+  }
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsXULPrototypeNode)
+  if (tmp->mType == nsXULPrototypeNode::eType_Element) {
+    nsXULPrototypeElement* elem = static_cast<nsXULPrototypeElement*>(tmp);
+    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mNodeInfo");
+    cb.NoteNativeChild(elem->mNodeInfo,
+                       NS_CYCLE_COLLECTION_PARTICIPANT(NodeInfo));
+    size_t i;
+    for (i = 0; i < elem->mAttributes.Length(); ++i) {
+      const nsAttrName& name = elem->mAttributes[i].mName;
+      if (!name.IsAtom()) {
+        NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb,
+                                           "mAttributes[i].mName.NodeInfo()");
+        cb.NoteNativeChild(name.NodeInfo(),
+                           NS_CYCLE_COLLECTION_PARTICIPANT(NodeInfo));
+      }
+    }
+    ImplCycleCollectionTraverse(cb, elem->mChildren, "mChildren");
+  }
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(nsXULPrototypeNode)
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+//----------------------------------------------------------------------
+//
+// nsXULPrototypeAttribute
+//
+
+nsXULPrototypeAttribute::~nsXULPrototypeAttribute() {
+  MOZ_COUNT_DTOR(nsXULPrototypeAttribute);
+}
+
+//----------------------------------------------------------------------
+//
+// nsXULPrototypeElement
+//
+
+nsresult nsXULPrototypeElement::Serialize(
+    nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  nsresult rv;
+
+  // Write basic prototype data
+  rv = aStream->Write32(mType);
+
+  // Write Node Info
+  int32_t index = aNodeInfos->IndexOf(mNodeInfo);
+  NS_ASSERTION(index >= 0, "unknown mozilla::dom::NodeInfo index");
+  nsresult tmp = aStream->Write32(index);
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+  // Write Attributes
+  tmp = aStream->Write32(mAttributes.Length());
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+  nsAutoString attributeValue;
+  size_t i;
+  for (i = 0; i < mAttributes.Length(); ++i) {
+    RefPtr<mozilla::dom::NodeInfo> ni;
+    if (mAttributes[i].mName.IsAtom()) {
+      ni = mNodeInfo->NodeInfoManager()->GetNodeInfo(
+          mAttributes[i].mName.Atom(), nullptr, kNameSpaceID_None,
+          nsINode::ATTRIBUTE_NODE);
+      NS_ASSERTION(ni, "the nodeinfo should already exist");
+    } else {
+      ni = mAttributes[i].mName.NodeInfo();
+    }
+
+    index = aNodeInfos->IndexOf(ni);
+    NS_ASSERTION(index >= 0, "unknown mozilla::dom::NodeInfo index");
+    tmp = aStream->Write32(index);
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+
+    mAttributes[i].mValue.ToString(attributeValue);
+    tmp = aStream->WriteWStringZ(attributeValue.get());
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+  }
+
+  // Now write children
+  tmp = aStream->Write32(uint32_t(mChildren.Length()));
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+  for (i = 0; i < mChildren.Length(); i++) {
+    nsXULPrototypeNode* child = mChildren[i].get();
+    switch (child->mType) {
+      case eType_Element:
+      case eType_Text:
+      case eType_PI:
+        tmp = child->Serialize(aStream, aProtoDoc, aNodeInfos);
+        if (NS_FAILED(tmp)) {
+          rv = tmp;
+        }
+        break;
+      case eType_Script:
+        tmp = aStream->Write32(child->mType);
+        if (NS_FAILED(tmp)) {
+          rv = tmp;
+        }
+        nsXULPrototypeScript* script =
+            static_cast<nsXULPrototypeScript*>(child);
+
+        tmp = aStream->Write8(script->mOutOfLine);
+        if (NS_FAILED(tmp)) {
+          rv = tmp;
+        }
+        if (!script->mOutOfLine) {
+          tmp = script->Serialize(aStream, aProtoDoc, aNodeInfos);
+          if (NS_FAILED(tmp)) {
+            rv = tmp;
+          }
+        } else {
+          tmp = aStream->WriteCompoundObject(script->mSrcURI,
+                                             NS_GET_IID(nsIURI), true);
+          if (NS_FAILED(tmp)) {
+            rv = tmp;
+          }
+
+          if (script->HasStencil()) {
+            // This may return NS_OK without muxing script->mSrcURI's
+            // data into the cache file, in the case where that
+            // muxed document is already there (written by a prior
+            // session, or by an earlier cache episode during this
+            // session).
+            tmp = script->SerializeOutOfLine(aStream, aProtoDoc);
+            if (NS_FAILED(tmp)) {
+              rv = tmp;
+            }
+          }
+        }
+        break;
+    }
+  }
+
+  return rv;
+}
+
+nsresult nsXULPrototypeElement::Deserialize(
+    nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    nsIURI* aDocumentURI,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  MOZ_ASSERT(aNodeInfos, "missing nodeinfo array");
+
+  // Read Node Info
+  uint32_t number = 0;
+  nsresult rv = aStream->Read32(&number);
+  if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+  mNodeInfo = aNodeInfos->SafeElementAt(number, nullptr);
+  if (!mNodeInfo) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (mNodeInfo->Equals(nsGkAtoms::parsererror) &&
+      mNodeInfo->NamespaceEquals(
+          nsDependentAtomString(nsGkAtoms::nsuri_parsererror))) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // Read Attributes
+  rv = aStream->Read32(&number);
+  if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+  int32_t attributes = int32_t(number);
+
+  if (attributes > 0) {
+    mAttributes.AppendElements(attributes);
+
+    nsAutoString attributeValue;
+    for (size_t i = 0; i < mAttributes.Length(); ++i) {
+      rv = aStream->Read32(&number);
+      if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+      mozilla::dom::NodeInfo* ni = aNodeInfos->SafeElementAt(number, nullptr);
+      if (!ni) {
+        return NS_ERROR_UNEXPECTED;
+      }
+
+      mAttributes[i].mName.SetTo(ni);
+
+      rv = aStream->ReadString(attributeValue);
+      if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+      rv = SetAttrAt(i, attributeValue, aDocumentURI);
+      if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+    }
+  }
+
+  rv = aStream->Read32(&number);
+  if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+  uint32_t numChildren = int32_t(number);
+
+  if (numChildren > 0) {
+    if (!mChildren.SetCapacity(numChildren, fallible)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    for (uint32_t i = 0; i < numChildren; i++) {
+      rv = aStream->Read32(&number);
+      if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+      Type childType = (Type)number;
+
+      RefPtr<nsXULPrototypeNode> child;
+
+      switch (childType) {
+        case eType_Element:
+          child = new nsXULPrototypeElement();
+          rv = child->Deserialize(aStream, aProtoDoc, aDocumentURI, aNodeInfos);
+          if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+          break;
+        case eType_Text:
+          child = new nsXULPrototypeText();
+          rv = child->Deserialize(aStream, aProtoDoc, aDocumentURI, aNodeInfos);
+          if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+          break;
+        case eType_PI:
+          child = new nsXULPrototypePI();
+          rv = child->Deserialize(aStream, aProtoDoc, aDocumentURI, aNodeInfos);
+          if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+          break;
+        case eType_Script: {
+          // language version/options obtained during deserialization.
+          RefPtr<nsXULPrototypeScript> script = new nsXULPrototypeScript(0);
+
+          rv = aStream->ReadBoolean(&script->mOutOfLine);
+          if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+          if (!script->mOutOfLine) {
+            rv = script->Deserialize(aStream, aProtoDoc, aDocumentURI,
+                                     aNodeInfos);
+            if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+          } else {
+            nsCOMPtr<nsISupports> supports;
+            rv = aStream->ReadObject(true, getter_AddRefs(supports));
+            if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+            script->mSrcURI = do_QueryInterface(supports);
+
+            rv = script->DeserializeOutOfLine(aStream, aProtoDoc);
+            if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+          }
+
+          child = std::move(script);
+          break;
+        }
+        default:
+          MOZ_ASSERT(false, "Unexpected child type!");
+          return NS_ERROR_UNEXPECTED;
+      }
+
+      MOZ_ASSERT(child, "Don't append null to mChildren");
+      MOZ_ASSERT(child->mType == childType);
+      mChildren.AppendElement(child);
+
+      // Oh dear. Something failed during the deserialization.
+      // We don't know what.  But likely consequences of failed
+      // deserializations included calls to |AbortCaching| which
+      // shuts down the cache and closes our streams.
+      // If that happens, next time through this loop, we die a messy
+      // death. So, let's just fail now, and propagate that failure
+      // upward so that the ChromeProtocolHandler knows it can't use
+      // a cached chrome channel for this.
+      if (NS_WARN_IF(NS_FAILED(rv))) return rv;
+    }
+  }
+
+  return rv;
+}
+
+nsresult nsXULPrototypeElement::SetAttrAt(uint32_t aPos,
+                                          const nsAString& aValue,
+                                          nsIURI* aDocumentURI) {
+  MOZ_ASSERT(aPos < mAttributes.Length(), "out-of-bounds");
+
+  // WARNING!!
+  // This code is largely duplicated in nsXULElement::SetAttr.
+  // Any changes should be made to both functions.
+
+  if (!mNodeInfo->NamespaceEquals(kNameSpaceID_XUL)) {
+    if (mNodeInfo->NamespaceEquals(kNameSpaceID_XHTML) &&
+        mAttributes[aPos].mName.Equals(nsGkAtoms::is)) {
+      // We still care about the is attribute set on HTML elements.
+      mAttributes[aPos].mValue.ParseAtom(aValue);
+      mIsAtom = mAttributes[aPos].mValue.GetAtomValue();
+
+      return NS_OK;
+    }
+
+    mAttributes[aPos].mValue.ParseStringOrAtom(aValue);
+
+    return NS_OK;
+  }
+
+  if (mAttributes[aPos].mName.Equals(nsGkAtoms::id) && !aValue.IsEmpty()) {
+    mHasIdAttribute = true;
+    // Store id as atom.
+    // id="" means that the element has no id. Not that it has
+    // emptystring as id.
+    mAttributes[aPos].mValue.ParseAtom(aValue);
+
+    return NS_OK;
+  } else if (mAttributes[aPos].mName.Equals(nsGkAtoms::is)) {
+    // Store is as atom.
+    mAttributes[aPos].mValue.ParseAtom(aValue);
+    mIsAtom = mAttributes[aPos].mValue.GetAtomValue();
+
+    return NS_OK;
+  } else if (mAttributes[aPos].mName.Equals(nsGkAtoms::_class)) {
+    mHasClassAttribute = true;
+    // Compute the element's class list
+    mAttributes[aPos].mValue.ParseAtomArray(aValue);
+
+    return NS_OK;
+  } else if (mAttributes[aPos].mName.Equals(nsGkAtoms::style)) {
+    mHasStyleAttribute = true;
+    // Parse the element's 'style' attribute
+
+    // This is basically duplicating what nsINode::NodePrincipal() does
+    nsIPrincipal* principal = mNodeInfo->NodeInfoManager()->DocumentPrincipal();
+    // XXX Get correct Base URI (need GetBaseURI on *prototype* element)
+    // TODO: If we implement Content Security Policy for chrome documents
+    // as has been discussed, the CSP should be checked here to see if
+    // inline styles are allowed to be applied.
+    // XXX No specific specs talk about xul and referrer policy, pass Unset
+    auto referrerInfo =
+        MakeRefPtr<ReferrerInfo>(aDocumentURI, ReferrerPolicy::_empty);
+    auto data = MakeRefPtr<URLExtraData>(aDocumentURI, referrerInfo, principal);
+    RefPtr<DeclarationBlock> declaration = DeclarationBlock::FromCssText(
+        aValue, data, eCompatibility_FullStandards, nullptr,
+        StyleCssRuleType::Style);
+    if (declaration) {
+      mAttributes[aPos].mValue.SetTo(declaration.forget(), &aValue);
+
+      return NS_OK;
+    }
+    // Don't abort if parsing failed, it could just be malformed css.
+  } else if (mAttributes[aPos].mName.Equals(nsGkAtoms::tabindex)) {
+    mAttributes[aPos].mValue.ParseIntValue(aValue);
+
+    return NS_OK;
+  }
+
+  mAttributes[aPos].mValue.ParseStringOrAtom(aValue);
+
+  return NS_OK;
+}
+
+void nsXULPrototypeElement::Unlink() {
+  mAttributes.Clear();
+  mChildren.Clear();
+}
+
+//----------------------------------------------------------------------
+//
+// nsXULPrototypeScript
+//
+
+nsXULPrototypeScript::nsXULPrototypeScript(uint32_t aLineNo)
+    : nsXULPrototypeNode(eType_Script),
+      mLineNo(aLineNo),
+      mSrcLoading(false),
+      mOutOfLine(true),
+      mSrcLoadWaiters(nullptr),
+      mStencil(nullptr) {}
+
+static nsresult WriteStencil(nsIObjectOutputStream* aStream, JSContext* aCx,
+                             JS::Stencil* aStencil) {
+  JS::TranscodeBuffer buffer;
+  JS::TranscodeResult code;
+  code = JS::EncodeStencil(aCx, aStencil, buffer);
+
+  if (code != JS::TranscodeResult::Ok) {
+    if (code == JS::TranscodeResult::Throw) {
+      JS_ClearPendingException(aCx);
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    MOZ_ASSERT(IsTranscodeFailureResult(code));
+    return NS_ERROR_FAILURE;
+  }
+
+  size_t size = buffer.length();
+  if (size > UINT32_MAX) {
+    return NS_ERROR_FAILURE;
+  }
+  nsresult rv = aStream->Write32(size);
+  if (NS_SUCCEEDED(rv)) {
+    // Ideally we could just pass "buffer" here.  See bug 1566574.
+    rv = aStream->WriteBytes(Span(buffer.begin(), size));
+  }
+
+  return rv;
+}
+
+static nsresult ReadStencil(nsIObjectInputStream* aStream, JSContext* aCx,
+                            const JS::ReadOnlyDecodeOptions& aOptions,
+                            JS::Stencil** aStencilOut) {
+  // We don't serialize mutedError-ness of scripts, which is fine as long as
+  // we only serialize system and XUL-y things. We can detect this by checking
+  // where the caller wants us to deserialize.
+  //
+  // CompilationScope() could theoretically GC, so get that out of the way
+  // before comparing to the cx global.
+  JSObject* loaderGlobal = xpc::CompilationScope();
+  MOZ_RELEASE_ASSERT(nsContentUtils::IsSystemCaller(aCx) ||
+                     JS::CurrentGlobalOrNull(aCx) == loaderGlobal);
+
+  uint32_t size;
+  nsresult rv = aStream->Read32(&size);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  char* data;
+  rv = aStream->ReadBytes(size, &data);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // The decoded stencil shouldn't borrow from the XDR buffer.
+  MOZ_ASSERT(!aOptions.borrowBuffer);
+  auto cleanupData = MakeScopeExit([&]() { free(data); });
+
+  JS::TranscodeRange range(reinterpret_cast<uint8_t*>(data), size);
+
+  {
+    JS::TranscodeResult code;
+    RefPtr<JS::Stencil> stencil;
+    code = JS::DecodeStencil(aCx, aOptions, range, getter_AddRefs(stencil));
+    if (code != JS::TranscodeResult::Ok) {
+      if (code == JS::TranscodeResult::Throw) {
+        JS_ClearPendingException(aCx);
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+
+      MOZ_ASSERT(IsTranscodeFailureResult(code));
+      return NS_ERROR_FAILURE;
+    }
+
+    stencil.forget(aStencilOut);
+  }
+
+  return rv;
+}
+
+void nsXULPrototypeScript::FillCompileOptions(JS::CompileOptions& aOptions,
+                                              const char* aFilename,
+                                              uint32_t aLineNo) {
+  // NOTE: This method shouldn't change any field which also exists in
+  //       JS::InstantiateOptions.  If such field is added,
+  //       nsXULPrototypeScript::InstantiateScript should also call this method.
+
+  // If the script was inline, tell the JS parser to save source for
+  // Function.prototype.toSource(). If it's out of line, we retrieve the
+  // source from the files on demand.
+  aOptions.setSourceIsLazy(mOutOfLine);
+
+  aOptions.setIntroductionType(mOutOfLine ? "srcScript" : "inlineScript")
+      .setFileAndLine(aFilename, mOutOfLine ? 1 : aLineNo);
+}
+
+nsresult nsXULPrototypeScript::Serialize(
+    nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  NS_ENSURE_TRUE(aProtoDoc, NS_ERROR_UNEXPECTED);
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(xpc::CompilationScope())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  NS_ASSERTION(!mSrcLoading || mSrcLoadWaiters != nullptr || !mStencil,
+               "script source still loading when serializing?!");
+  if (!mStencil) return NS_ERROR_FAILURE;
+
+  // Write basic prototype data
+  nsresult rv;
+  rv = aStream->Write32(mLineNo);
+  if (NS_FAILED(rv)) return rv;
+
+  JSContext* cx = jsapi.cx();
+  MOZ_ASSERT(xpc::CompilationScope() == JS::CurrentGlobalOrNull(cx));
+
+  return WriteStencil(aStream, cx, mStencil);
+}
+
+nsresult nsXULPrototypeScript::SerializeOutOfLine(
+    nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc) {
+  if (!mSrcURI->SchemeIs("chrome"))
+    // Don't cache scripts that don't come from chrome uris.
+    return NS_ERROR_NOT_IMPLEMENTED;
+
+  nsXULPrototypeCache* cache = nsXULPrototypeCache::GetInstance();
+  if (!cache) return NS_ERROR_OUT_OF_MEMORY;
+
+  NS_ASSERTION(cache->IsEnabled(),
+               "writing to the cache file, but the XUL cache is off?");
+  bool exists;
+  cache->HasScript(mSrcURI, &exists);
+
+  /* return will be NS_OK from GetAsciiSpec.
+   * that makes no sense.
+   * nor does returning NS_OK from HasMuxedDocument.
+   * XXX return something meaningful.
+   */
+  if (exists) return NS_OK;
+
+  nsCOMPtr<nsIObjectOutputStream> oos;
+  nsresult rv = cache->GetScriptOutputStream(mSrcURI, getter_AddRefs(oos));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsresult tmp = Serialize(oos, aProtoDoc, nullptr);
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+  tmp = cache->FinishScriptOutputStream(mSrcURI);
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+  if (NS_FAILED(rv)) cache->AbortCaching();
+  return rv;
+}
+
+nsresult nsXULPrototypeScript::Deserialize(
+    nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    nsIURI* aDocumentURI,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  nsresult rv;
+  NS_ASSERTION(!mSrcLoading || mSrcLoadWaiters != nullptr || !mStencil,
+               "prototype script not well-initialized when deserializing?!");
+
+  // Read basic prototype data
+  rv = aStream->Read32(&mLineNo);
+  if (NS_FAILED(rv)) return rv;
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(xpc::CompilationScope())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  JSContext* cx = jsapi.cx();
+
+  JS::DecodeOptions options;
+  RefPtr<JS::Stencil> newStencil;
+  rv = ReadStencil(aStream, cx, options, getter_AddRefs(newStencil));
+  NS_ENSURE_SUCCESS(rv, rv);
+  Set(newStencil);
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeScript::DeserializeOutOfLine(
+    nsIObjectInputStream* aInput, nsXULPrototypeDocument* aProtoDoc) {
+  // Keep track of failure via rv, so we can
+  // AbortCaching if things look bad.
+  nsresult rv = NS_OK;
+  nsXULPrototypeCache* cache = nsXULPrototypeCache::GetInstance();
+
+  nsCOMPtr<nsIObjectInputStream> objectInput = aInput;
+  if (cache) {
+    bool useXULCache = true;
+    if (mSrcURI) {
+      // NB: we must check the XUL script cache early, to avoid
+      // multiple deserialization attempts for a given script.
+      // Note that PrototypeDocumentContentSink::LoadScript
+      // checks the XUL script cache too, in order to handle the
+      // serialization case.
+      //
+      // We need do this only for <script src='strres.js'> and the
+      // like, i.e., out-of-line scripts that are included by several
+      // different XUL documents stored in the cache file.
+      useXULCache = cache->IsEnabled();
+
+      if (useXULCache) {
+        RefPtr<JS::Stencil> newStencil = cache->GetStencil(mSrcURI);
+        if (newStencil) {
+          Set(newStencil);
+        }
+      }
+    }
+
+    if (!mStencil) {
+      if (mSrcURI) {
+        rv = cache->GetScriptInputStream(mSrcURI, getter_AddRefs(objectInput));
+      }
+      // If !mSrcURI, we have an inline script. We shouldn't have
+      // to do anything else in that case, I think.
+
+      // We do reflect errors into rv, but our caller may want to
+      // ignore our return value, because mStencil will be null
+      // after any error, and that suffices to cause the script to
+      // be reloaded (from the src= URI, if any) and recompiled.
+      // We're better off slow-loading than bailing out due to a
+      // error.
+      if (NS_SUCCEEDED(rv))
+        rv = Deserialize(objectInput, aProtoDoc, nullptr, nullptr);
+
+      if (NS_SUCCEEDED(rv)) {
+        if (useXULCache && mSrcURI && mSrcURI->SchemeIs("chrome")) {
+          cache->PutStencil(mSrcURI, GetStencil());
+        }
+        cache->FinishScriptInputStream(mSrcURI);
+      } else {
+        // If mSrcURI is not in the cache,
+        // rv will be NS_ERROR_NOT_AVAILABLE and we'll try to
+        // update the cache file to hold a serialization of
+        // this script, once it has finished loading.
+        if (rv != NS_ERROR_NOT_AVAILABLE) cache->AbortCaching();
+      }
+    }
+  }
+  return rv;
+}
+
+#ifdef DEBUG
+static void CheckErrorsAndWarnings(JS::FrontendContext* aFc,
+                                   const JS::ReadOnlyCompileOptions& aOptions) {
+  if (JS::HadFrontendErrors(aFc)) {
+    const JSErrorReport* report = JS::GetFrontendErrorReport(aFc, aOptions);
+    if (report) {
+      const char* message = "<unknown>";
+      const char* filename = "<unknown>";
+
+      if (report->message().c_str()) {
+        message = report->message().c_str();
+      }
+      if (report->filename.c_str()) {
+        filename = report->filename.c_str();
+      }
+
+      NS_WARNING(
+          nsPrintfCString(
+              "Had compilation error in ScriptCompileTask: %s at %s:%u:%u",
+              message, filename, report->lineno,
+              report->column.oneOriginValue())
+              .get());
+    }
+
+    if (JS::HadFrontendOverRecursed(aFc)) {
+      NS_WARNING("Had over recursed in ScriptCompileTask");
+    }
+
+    if (JS::HadFrontendOutOfMemory(aFc)) {
+      NS_WARNING("Had out of memory in ScriptCompileTask");
+    }
+
+    if (JS::HadFrontendAllocationOverflow(aFc)) {
+      NS_WARNING("Had allocation overflow in ScriptCompileTask");
+    }
+  }
+
+  size_t count = JS::GetFrontendWarningCount(aFc);
+  for (size_t i = 0; i < count; i++) {
+    const JSErrorReport* report = JS::GetFrontendWarningAt(aFc, i, aOptions);
+
+    const char* message = "<unknown>";
+    const char* filename = "<unknown>";
+
+    if (report->message().c_str()) {
+      message = report->message().c_str();
+    }
+    if (report->filename.c_str()) {
+      filename = report->filename.c_str();
+    }
+
+    NS_WARNING(
+        nsPrintfCString(
+            "Had compilation warning in ScriptCompileTask: %s at %s:%u:%u",
+            message, filename, report->lineno, report->column.oneOriginValue())
+            .get());
+  }
+}
+#endif
+
+class ScriptCompileTask final : public Task {
+ public:
+  explicit ScriptCompileTask(UniquePtr<Utf8Unit[], JS::FreePolicy>&& aText,
+                             size_t aTextLength)
+      : Task(Kind::OffMainThreadOnly, EventQueuePriority::Normal),
+        mOptions(JS::OwningCompileOptions::ForFrontendContext()),
+        mText(std::move(aText)),
+        mTextLength(aTextLength) {}
+
+  ~ScriptCompileTask() {
+    if (mFrontendContext) {
+      JS::DestroyFrontendContext(mFrontendContext);
+    }
+  }
+
+  nsresult Init(JS::CompileOptions& aOptions) {
+    mFrontendContext = JS::NewFrontendContext();
+    if (!mFrontendContext) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (!mOptions.copy(mFrontendContext, aOptions)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    return NS_OK;
+  }
+
+ private:
+  void Compile() {
+    // NOTE: The stack limit must be set from the same thread that compiles.
+    size_t stackSize = TaskController::GetThreadStackSize();
+    JS::SetNativeStackQuota(mFrontendContext,
+                            JS::ThreadStackQuotaForSize(stackSize));
+
+    JS::SourceText<Utf8Unit> srcBuf;
+    if (NS_WARN_IF(!srcBuf.init(mFrontendContext, mText.get(), mTextLength,
+                                JS::SourceOwnership::Borrowed))) {
+      return;
+    }
+
+    JS::CompilationStorage compileStorage;
+    mStencil = JS::CompileGlobalScriptToStencil(mFrontendContext, mOptions,
+                                                srcBuf, compileStorage);
+#ifdef DEBUG
+    // Chrome-privileged code shouldn't have any compilation error.
+    CheckErrorsAndWarnings(mFrontendContext, mOptions);
+    MOZ_ASSERT(mStencil);
+#endif
+  }
+
+ public:
+  TaskResult Run() override {
+    Compile();
+    return TaskResult::Complete;
+  }
+
+  already_AddRefed<JS::Stencil> StealStencil() { return mStencil.forget(); }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  bool GetName(nsACString& aName) override {
+    aName.AssignLiteral("ScriptCompileTask");
+    return true;
+  }
+#endif
+
+ private:
+  // Owning-pointer for the context associated with the script compilation.
+  //
+  // The context is allocated on main thread in Init method, and is freed on
+  // any thread in the destructor.
+  JS::FrontendContext* mFrontendContext = nullptr;
+
+  JS::OwningCompileOptions mOptions;
+
+  RefPtr<JS::Stencil> mStencil;
+
+  // The source text for this compilation.
+  UniquePtr<Utf8Unit[], JS::FreePolicy> mText;
+  size_t mTextLength;
+};
+
+class NotifyOffThreadScriptCompletedTask : public Task {
+ public:
+  NotifyOffThreadScriptCompletedTask(nsIOffThreadScriptReceiver* aReceiver,
+                                     ScriptCompileTask* aCompileTask)
+      : Task(Kind::MainThreadOnly, EventQueuePriority::Normal),
+        mReceiver(aReceiver),
+        mCompileTask(aCompileTask) {}
+
+  TaskResult Run() override {
+    MOZ_ASSERT(NS_IsMainThread());
+
+    if (PastShutdownPhase(ShutdownPhase::XPCOMShutdownFinal)) {
+      return TaskResult::Complete;
+    }
+
+    RefPtr<JS::Stencil> stencil = mCompileTask->StealStencil();
+    mCompileTask = nullptr;
+
+    (void)mReceiver->OnScriptCompileComplete(
+        stencil, stencil ? NS_OK : NS_ERROR_FAILURE);
+
+    return TaskResult::Complete;
+  }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  bool GetName(nsACString& aName) override {
+    aName.AssignLiteral("NotifyOffThreadScriptCompletedTask");
+    return true;
+  }
+#endif
+
+ private:
+  // NOTE:
+  // This field is main-thread only, and this task shouldn't be freed off
+  // main thread.
+  //
+  // This is guaranteed by not having off-thread tasks which depends on this
+  // task, or any other pointer from off-thread task to this task, because
+  // otherwise the off-thread task's mDependencies can be the last reference,
+  // which results in freeing this task off main thread.
+  //
+  // If such task is added, this field must be moved to separate storage.
+  nsCOMPtr<nsIOffThreadScriptReceiver> mReceiver;
+
+  RefPtr<ScriptCompileTask> mCompileTask;
+};
+
+nsresult StartOffThreadCompile(JS::CompileOptions& aOptions,
+                               UniquePtr<Utf8Unit[], JS::FreePolicy>&& aText,
+                               size_t aTextLength,
+                               nsIOffThreadScriptReceiver* aOffThreadReceiver) {
+  RefPtr<ScriptCompileTask> compileTask =
+      new ScriptCompileTask(std::move(aText), aTextLength);
+
+  RefPtr<NotifyOffThreadScriptCompletedTask> notifyTask =
+      new NotifyOffThreadScriptCompletedTask(aOffThreadReceiver, compileTask);
+
+  nsresult rv = compileTask->Init(aOptions);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  notifyTask->AddDependency(compileTask.get());
+
+  TaskController::Get()->AddTask(compileTask.forget());
+  TaskController::Get()->AddTask(notifyTask.forget());
+
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeScript::Compile(const char16_t* aText,
+                                       size_t aTextLength, nsIURI* aURI,
+                                       uint32_t aLineNo, Document* aDocument) {
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(xpc::CompilationScope())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  JSContext* cx = jsapi.cx();
+
+  JS::SourceText<char16_t> srcBuf;
+  if (NS_WARN_IF(!srcBuf.init(cx, aText, aTextLength,
+                              JS::SourceOwnership::Borrowed))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsAutoCString urlspec;
+  nsresult rv = aURI->GetSpec(urlspec);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  JS::CompileOptions options(cx);
+  FillCompileOptions(options, urlspec.get(), aLineNo);
+
+  RefPtr<JS::Stencil> stencil =
+      JS::CompileGlobalScriptToStencil(cx, options, srcBuf);
+  if (!stencil) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  Set(stencil);
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeScript::CompileMaybeOffThread(
+    mozilla::UniquePtr<mozilla::Utf8Unit[], JS::FreePolicy>&& aText,
+    size_t aTextLength, nsIURI* aURI, uint32_t aLineNo, Document* aDocument,
+    nsIOffThreadScriptReceiver* aOffThreadReceiver) {
+  MOZ_ASSERT(aOffThreadReceiver);
+
+  nsAutoCString urlspec;
+  nsresult rv = aURI->GetSpec(urlspec);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(xpc::CompilationScope())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  JSContext* cx = jsapi.cx();
+
+  JS::CompileOptions options(cx);
+  FillCompileOptions(options, urlspec.get(), aLineNo);
+
+  // TODO: This uses the same heuristics and the same threshold as the
+  //       JS::CanDecodeOffThread API, but the heuristics needs to be updated
+  //       to reflect the change regarding the Stencil API, and also the thread
+  //       management on the consumer side (bug 1840831).
+  static constexpr size_t OffThreadMinimumTextLength = 5 * 1000;
+
+  if (StaticPrefs::javascript_options_parallel_parsing() &&
+      aTextLength >= OffThreadMinimumTextLength) {
+    rv = StartOffThreadCompile(options, std::move(aText), aTextLength,
+                               aOffThreadReceiver);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  } else {
+    JS::SourceText<Utf8Unit> srcBuf;
+    if (NS_WARN_IF(!srcBuf.init(cx, aText.get(), aTextLength,
+                                JS::SourceOwnership::Borrowed))) {
+      return NS_ERROR_FAILURE;
+    }
+
+    RefPtr<JS::Stencil> stencil =
+        JS::CompileGlobalScriptToStencil(cx, options, srcBuf);
+    if (!stencil) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+    Set(stencil);
+  }
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeScript::InstantiateScript(
+    JSContext* aCx, JS::MutableHandle<JSScript*> aScript) {
+  MOZ_ASSERT(mStencil);
+
+  JS::CompileOptions options(aCx);
+  JS::InstantiateOptions instantiateOptions(options);
+  aScript.set(JS::InstantiateGlobalStencil(aCx, instantiateOptions, mStencil));
+  if (!aScript) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+void nsXULPrototypeScript::Set(JS::Stencil* aStencil) { mStencil = aStencil; }
+
+//----------------------------------------------------------------------
+//
+// nsXULPrototypeText
+//
+
+nsresult nsXULPrototypeText::Serialize(
+    nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  nsresult rv;
+
+  // Write basic prototype data
+  rv = aStream->Write32(mType);
+
+  nsresult tmp = aStream->WriteWStringZ(mValue.get());
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+  return rv;
+}
+
+nsresult nsXULPrototypeText::Deserialize(
+    nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    nsIURI* aDocumentURI,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  nsresult rv = aStream->ReadString(mValue);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------
+//
+// nsXULPrototypePI
+//
+
+nsresult nsXULPrototypePI::Serialize(
+    nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  nsresult rv;
+
+  // Write basic prototype data
+  rv = aStream->Write32(mType);
+
+  nsresult tmp = aStream->WriteWStringZ(mTarget.get());
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+  tmp = aStream->WriteWStringZ(mData.get());
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+  return rv;
+}
+
+nsresult nsXULPrototypePI::Deserialize(
+    nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+    nsIURI* aDocumentURI,
+    const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) {
+  nsresult rv;
+
+  rv = aStream->ReadString(mTarget);
+  if (NS_FAILED(rv)) return rv;
+  rv = aStream->ReadString(mData);
+  if (NS_FAILED(rv)) return rv;
+
+  return rv;
+}
diff --git a/dom/xul/nsXULElement.h b/dom/xul/nsXULElement.h
new file mode 100644
index 0000000000..4002ec63dc
--- /dev/null
+++ b/dom/xul/nsXULElement.h
@@ -0,0 +1,568 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+/*
+
+  The base XUL element class and associates.
+
+*/
+
+#ifndef nsXULElement_h__
+#define nsXULElement_h__
+
+#include <stdint.h>
+#include <stdio.h>
+#include "ErrorList.h"
+#include "js/experimental/JSStencil.h"
+#include "js/RootingAPI.h"
+#include "js/SourceText.h"
+#include "js/TracingAPI.h"
+#include "js/TypeDecls.h"
+#include "js/Utility.h"  // JS::FreePolicy
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/BasicEvents.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/dom/DOMString.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/FragmentOrElement.h"
+#include "mozilla/dom/FromParser.h"
+#include "mozilla/dom/NameSpaceConstants.h"
+#include "mozilla/dom/NodeInfo.h"
+#include "nsAtom.h"
+#include "nsAttrName.h"
+#include "nsAttrValue.h"
+#include "nsCOMPtr.h"
+#include "nsCaseTreatment.h"
+#include "nsChangeHint.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsGkAtoms.h"
+#include "nsIContent.h"
+#include "nsINode.h"
+#include "nsISupports.h"
+#include "nsLiteralString.h"
+#include "nsString.h"
+#include "nsStyledElement.h"
+#include "nsTArray.h"
+#include "nsTLiteralString.h"
+#include "nscore.h"
+
+class JSObject;
+class nsIControllers;
+class nsIObjectInputStream;
+class nsIObjectOutputStream;
+class nsIOffThreadScriptReceiver;
+class nsIPrincipal;
+class nsIURI;
+class nsXULPrototypeDocument;
+class nsXULPrototypeNode;
+struct JSContext;
+
+using nsPrototypeArray = nsTArray<RefPtr<nsXULPrototypeNode>>;
+
+namespace JS {
+class CompileOptions;
+}
+
+namespace mozilla {
+class ErrorResult;
+class EventChainPreVisitor;
+class EventListenerManager;
+namespace css {
+class StyleRule;
+}  // namespace css
+namespace dom {
+class Document;
+class HTMLIFrameElement;
+class PrototypeDocumentContentSink;
+enum class CallerType : uint32_t;
+}  // namespace dom
+}  // namespace mozilla
+
+////////////////////////////////////////////////////////////////////////
+
+#ifdef XUL_PROTOTYPE_ATTRIBUTE_METERING
+#  define XUL_PROTOTYPE_ATTRIBUTE_METER(counter) \
+    (nsXULPrototypeAttribute::counter++)
+#else
+#  define XUL_PROTOTYPE_ATTRIBUTE_METER(counter) ((void)0)
+#endif
+
+/**
+
+  A prototype attribute for an nsXULPrototypeElement.
+
+ */
+
+class nsXULPrototypeAttribute {
+ public:
+  nsXULPrototypeAttribute()
+      : mName(nsGkAtoms::id)  // XXX this is a hack, but names have to have a
+                              // value
+  {
+    XUL_PROTOTYPE_ATTRIBUTE_METER(gNumAttributes);
+    MOZ_COUNT_CTOR(nsXULPrototypeAttribute);
+  }
+
+  ~nsXULPrototypeAttribute();
+
+  nsAttrName mName;
+  nsAttrValue mValue;
+
+#ifdef XUL_PROTOTYPE_ATTRIBUTE_METERING
+  static uint32_t gNumElements;
+  static uint32_t gNumAttributes;
+  static uint32_t gNumCacheTests;
+  static uint32_t gNumCacheHits;
+  static uint32_t gNumCacheSets;
+  static uint32_t gNumCacheFills;
+#endif /* !XUL_PROTOTYPE_ATTRIBUTE_METERING */
+};
+
+/**
+
+  A prototype content model element that holds the "primordial" values
+  that have been parsed from the original XUL document.
+
+ */
+
+class nsXULPrototypeNode {
+ public:
+  enum Type { eType_Element, eType_Script, eType_Text, eType_PI };
+
+  Type mType;
+
+  virtual nsresult Serialize(
+      nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) = 0;
+  virtual nsresult Deserialize(
+      nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      nsIURI* aDocumentURI,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) = 0;
+
+  /**
+   * The prototype document must call ReleaseSubtree when it is going
+   * away.  This makes the parents through the tree stop owning their
+   * children, whether or not the parent's reference count is zero.
+   * Individual elements may still own individual prototypes, but
+   * those prototypes no longer remember their children to allow them
+   * to be constructed.
+   */
+  virtual void ReleaseSubtree() {}
+
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(nsXULPrototypeNode)
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(nsXULPrototypeNode)
+
+ protected:
+  explicit nsXULPrototypeNode(Type aType) : mType(aType) {}
+  virtual ~nsXULPrototypeNode() = default;
+};
+
+class nsXULPrototypeElement : public nsXULPrototypeNode {
+ public:
+  explicit nsXULPrototypeElement(mozilla::dom::NodeInfo* aNodeInfo = nullptr)
+      : nsXULPrototypeNode(eType_Element),
+        mNodeInfo(aNodeInfo),
+        mHasIdAttribute(false),
+        mHasClassAttribute(false),
+        mHasStyleAttribute(false),
+        mIsAtom(nullptr) {}
+
+ private:
+  virtual ~nsXULPrototypeElement() { Unlink(); }
+
+ public:
+  virtual void ReleaseSubtree() override {
+    for (int32_t i = mChildren.Length() - 1; i >= 0; i--) {
+      if (mChildren[i].get()) mChildren[i]->ReleaseSubtree();
+    }
+    mChildren.Clear();
+    nsXULPrototypeNode::ReleaseSubtree();
+  }
+
+  virtual nsresult Serialize(
+      nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+  virtual nsresult Deserialize(
+      nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      nsIURI* aDocumentURI,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+
+  nsresult SetAttrAt(uint32_t aPos, const nsAString& aValue,
+                     nsIURI* aDocumentURI);
+
+  void Unlink();
+
+  nsPrototypeArray mChildren;
+
+  RefPtr<mozilla::dom::NodeInfo> mNodeInfo;
+
+  uint32_t mHasIdAttribute : 1;
+  uint32_t mHasClassAttribute : 1;
+  uint32_t mHasStyleAttribute : 1;
+  nsTArray<nsXULPrototypeAttribute> mAttributes;  // [OWNER]
+  RefPtr<nsAtom> mIsAtom;
+};
+
+class nsXULPrototypeScript : public nsXULPrototypeNode {
+ public:
+  explicit nsXULPrototypeScript(uint32_t aLineNo);
+
+ private:
+  virtual ~nsXULPrototypeScript() = default;
+
+  void FillCompileOptions(JS::CompileOptions& aOptions, const char* aFilename,
+                          uint32_t aLineNo);
+
+ public:
+  virtual nsresult Serialize(
+      nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+  nsresult SerializeOutOfLine(nsIObjectOutputStream* aStream,
+                              nsXULPrototypeDocument* aProtoDoc);
+  virtual nsresult Deserialize(
+      nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      nsIURI* aDocumentURI,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+  nsresult DeserializeOutOfLine(nsIObjectInputStream* aInput,
+                                nsXULPrototypeDocument* aProtoDoc);
+
+  // Compile given JS source text synchronously.
+  //
+  // This method doesn't take the ownership of aText, but borrows during the
+  // compilation.
+  //
+  // If successfully compiled, `HasStencil()` returns true.
+  nsresult Compile(const char16_t* aText, size_t aTextLength, nsIURI* aURI,
+                   uint32_t aLineNo, mozilla::dom::Document* aDocument);
+
+  // Compile given JS source text possibly in off-thread.
+  //
+  // This method takes the ownership of aText.
+  //
+  // If this doesn't use off-thread compilation and successfully compiled,
+  // `HasStencil()` returns true.
+  //
+  // If this uses off-thread compilation, `HasStencil()` returns false, and
+  // once the compilation finishes, aOffThreadReceiver gets notified with the
+  // compiled stencil.  The callback is responsible for calling `Set()` with
+  // the stencil.
+  nsresult CompileMaybeOffThread(
+      mozilla::UniquePtr<mozilla::Utf8Unit[], JS::FreePolicy>&& aText,
+      size_t aTextLength, nsIURI* aURI, uint32_t aLineNo,
+      mozilla::dom::Document* aDocument,
+      nsIOffThreadScriptReceiver* aOffThreadReceiver);
+
+  void Set(JS::Stencil* aStencil);
+
+  bool HasStencil() { return mStencil; }
+
+  JS::Stencil* GetStencil() { return mStencil.get(); }
+
+  nsresult InstantiateScript(JSContext* aCx,
+                             JS::MutableHandle<JSScript*> aScript);
+
+  nsCOMPtr<nsIURI> mSrcURI;
+  uint32_t mLineNo;
+  bool mSrcLoading;
+  bool mOutOfLine;
+  mozilla::dom::PrototypeDocumentContentSink*
+      mSrcLoadWaiters;  // [OWNER] but not COMPtr
+ private:
+  RefPtr<JS::Stencil> mStencil;
+};
+
+class nsXULPrototypeText : public nsXULPrototypeNode {
+ public:
+  nsXULPrototypeText() : nsXULPrototypeNode(eType_Text) {}
+
+ private:
+  virtual ~nsXULPrototypeText() = default;
+
+ public:
+  virtual nsresult Serialize(
+      nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+  virtual nsresult Deserialize(
+      nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      nsIURI* aDocumentURI,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+
+  nsString mValue;
+};
+
+class nsXULPrototypePI : public nsXULPrototypeNode {
+ public:
+  nsXULPrototypePI() : nsXULPrototypeNode(eType_PI) {}
+
+ private:
+  virtual ~nsXULPrototypePI() = default;
+
+ public:
+  virtual nsresult Serialize(
+      nsIObjectOutputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+  virtual nsresult Deserialize(
+      nsIObjectInputStream* aStream, nsXULPrototypeDocument* aProtoDoc,
+      nsIURI* aDocumentURI,
+      const nsTArray<RefPtr<mozilla::dom::NodeInfo>>* aNodeInfos) override;
+
+  nsString mTarget;
+  nsString mData;
+};
+
+////////////////////////////////////////////////////////////////////////
+
+/**
+
+  The XUL element.
+
+ */
+
+#define XUL_ELEMENT_FLAG_BIT(n_) \
+  NODE_FLAG_BIT(ELEMENT_TYPE_SPECIFIC_BITS_OFFSET + (n_))
+
+// XUL element specific bits
+enum {
+  XUL_ELEMENT_HAS_CONTENTMENU_LISTENER = XUL_ELEMENT_FLAG_BIT(0),
+  XUL_ELEMENT_HAS_POPUP_LISTENER = XUL_ELEMENT_FLAG_BIT(1)
+};
+
+ASSERT_NODE_FLAGS_SPACE(ELEMENT_TYPE_SPECIFIC_BITS_OFFSET + 2);
+
+#undef XUL_ELEMENT_FLAG_BIT
+
+class nsXULElement : public nsStyledElement {
+ protected:
+  using Document = mozilla::dom::Document;
+
+  // Use Construct to construct elements instead of this constructor.
+  explicit nsXULElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+ public:
+  using Element::Blur;
+  using Element::Focus;
+
+  static nsresult CreateFromPrototype(nsXULPrototypeElement* aPrototype,
+                                      Document* aDocument, bool aIsScriptable,
+                                      bool aIsRoot,
+                                      mozilla::dom::Element** aResult);
+
+  // This is the constructor for nsXULElements.
+  static nsXULElement* Construct(
+      already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+  NS_IMPL_FROMNODE(nsXULElement, kNameSpaceID_XUL)
+
+  // nsISupports
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(nsXULElement, nsStyledElement)
+
+  // This doesn't work on XUL elements! You probably want
+  // GetXULBoolAttr(nsGkAtoms::disabled) or so.
+  // TODO(emilio): Maybe we should unify HTML and XUL here.
+  bool IsDisabled() const = delete;
+
+  // nsINode
+  void GetEventTargetParent(mozilla::EventChainPreVisitor& aVisitor) override;
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  virtual nsresult PreHandleEvent(
+      mozilla::EventChainVisitor& aVisitor) override;
+  // nsIContent
+  virtual nsresult BindToTree(BindContext&, nsINode& aParent) override;
+  virtual void UnbindFromTree(bool aNullParent) override;
+  virtual void DestroyContent() override;
+  virtual void DoneAddingChildren(bool aHaveNotified) override;
+
+#ifdef MOZ_DOM_LIST
+  virtual void List(FILE* out, int32_t aIndent) const override;
+  virtual void DumpContent(FILE* out, int32_t aIndent,
+                           bool aDumpAll) const override {}
+#endif
+
+  MOZ_CAN_RUN_SCRIPT bool HasMenu();
+  MOZ_CAN_RUN_SCRIPT void OpenMenu(bool aOpenFlag);
+
+  MOZ_CAN_RUN_SCRIPT
+  virtual mozilla::Result<bool, nsresult> PerformAccesskey(
+      bool aKeyCausesActivation, bool aIsTrustedEvent) override;
+  MOZ_CAN_RUN_SCRIPT void ClickWithInputSource(uint16_t aInputSource,
+                                               bool aIsTrustedEvent);
+  struct XULFocusability {
+    bool mDefaultFocusable = false;
+    mozilla::Maybe<bool> mForcedFocusable;
+    mozilla::Maybe<int32_t> mForcedTabIndexIfFocusable;
+
+    static XULFocusability NeverFocusable() {
+      return {false, mozilla::Some(false), mozilla::Some(-1)};
+    }
+  };
+  XULFocusability GetXULFocusability(bool aWithMouse);
+  Focusable IsFocusableWithoutStyle(bool aWithMouse) override;
+
+  NS_IMETHOD_(bool) IsAttributeMapped(const nsAtom* aAttribute) const override;
+
+  virtual nsresult Clone(mozilla::dom::NodeInfo*,
+                         nsINode** aResult) const override;
+
+  virtual bool IsEventAttributeNameInternal(nsAtom* aName) override;
+
+  using DOMString = mozilla::dom::DOMString;
+  void GetXULAttr(nsAtom* aName, DOMString& aResult) const {
+    GetAttr(aName, aResult);
+  }
+  void SetXULAttr(nsAtom* aName, const nsAString& aValue,
+                  mozilla::ErrorResult& aError) {
+    SetAttr(aName, aValue, aError);
+  }
+  bool GetXULBoolAttr(nsAtom* aName) const {
+    return AttrValueIs(kNameSpaceID_None, aName, u"true"_ns, eCaseMatters);
+  }
+  void SetXULBoolAttr(nsAtom* aName, bool aValue,
+                      mozilla::ErrorResult& aError) {
+    if (aValue) {
+      SetAttr(aName, u"true"_ns, aError);
+    } else {
+      UnsetAttr(aName, aError);
+    }
+  }
+
+  // WebIDL API
+  bool Autofocus() const { return BoolAttrIsTrue(nsGkAtoms::autofocus); }
+  void SetAutofocus(bool aAutofocus, ErrorResult& aRv) {
+    SetXULBoolAttr(nsGkAtoms::autofocus, aAutofocus, aRv);
+  }
+  bool Hidden() const { return BoolAttrIsTrue(nsGkAtoms::hidden); }
+  void SetHidden(bool aHidden) {
+    SetXULBoolAttr(nsGkAtoms::hidden, aHidden, mozilla::IgnoreErrors());
+  }
+  bool Collapsed() const { return BoolAttrIsTrue(nsGkAtoms::collapsed); }
+  void SetCollapsed(bool aCollapsed) {
+    SetXULBoolAttr(nsGkAtoms::collapsed, aCollapsed, mozilla::IgnoreErrors());
+  }
+  void GetObserves(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::observes, aValue);
+  }
+  void SetObserves(const nsAString& aValue, mozilla::ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::observes, aValue, rv);
+  }
+  void GetMenu(DOMString& aValue) const { GetXULAttr(nsGkAtoms::menu, aValue); }
+  void SetMenu(const nsAString& aValue, mozilla::ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::menu, aValue, rv);
+  }
+  void GetContextMenu(DOMString& aValue) {
+    GetXULAttr(nsGkAtoms::contextmenu, aValue);
+  }
+  void SetContextMenu(const nsAString& aValue, mozilla::ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::contextmenu, aValue, rv);
+  }
+  void GetTooltip(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::tooltip, aValue);
+  }
+  void SetTooltip(const nsAString& aValue, mozilla::ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::tooltip, aValue, rv);
+  }
+  void GetTooltipText(DOMString& aValue) const {
+    GetXULAttr(nsGkAtoms::tooltiptext, aValue);
+  }
+  void SetTooltipText(const nsAString& aValue, mozilla::ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::tooltiptext, aValue, rv);
+  }
+  void GetSrc(DOMString& aValue) const { GetXULAttr(nsGkAtoms::src, aValue); }
+  void SetSrc(const nsAString& aValue, mozilla::ErrorResult& rv) {
+    SetXULAttr(nsGkAtoms::src, aValue, rv);
+  }
+  nsIControllers* GetControllers(mozilla::ErrorResult& rv);
+  // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void Click(mozilla::dom::CallerType aCallerType);
+  // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void DoCommand();
+  // Style() inherited from nsStyledElement
+
+  nsINode* GetScopeChainParent() const override {
+    // For XUL, the parent is the parent element, if any
+    Element* parent = GetParentElement();
+    return parent ? parent : nsStyledElement::GetScopeChainParent();
+  }
+
+  bool IsInteractiveHTMLContent() const override;
+
+ protected:
+  ~nsXULElement();
+
+  // This can be removed if EnsureContentsGenerated dies.
+  friend class nsNSElementTearoff;
+
+  // Implementation methods
+  nsresult EnsureContentsGenerated(void) const;
+
+  nsresult AddPopupListener(nsAtom* aName);
+
+  /**
+   * Abandon our prototype linkage, and copy all attributes locally
+   */
+  nsresult MakeHeavyweight(nsXULPrototypeElement* aPrototype);
+
+  void BeforeSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                     const nsAttrValue* aValue, bool aNotify) override;
+  void AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
+                    const nsAttrValue* aValue, const nsAttrValue* aOldValue,
+                    nsIPrincipal* aSubjectPrincipal, bool aNotify) override;
+
+  bool ParseAttribute(int32_t aNamespaceID, nsAtom* aAttribute,
+                      const nsAString& aValue,
+                      nsIPrincipal* aMaybeScriptedPrincipal,
+                      nsAttrValue& aResult) override;
+
+  mozilla::EventListenerManager* GetEventListenerManagerForAttr(
+      nsAtom* aAttrName, bool* aDefer) override;
+
+  /**
+   * Add a listener for the specified attribute, if appropriate.
+   */
+  void AddListenerForAttributeIfNeeded(const nsAttrName& aName);
+  void AddListenerForAttributeIfNeeded(nsAtom* aLocalName);
+
+ protected:
+  void AddTooltipSupport();
+  void RemoveTooltipSupport();
+
+  // Internal accessor. This shadows the 'Slots', and returns
+  // appropriate value.
+  nsIControllers* Controllers() {
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
+    return slots ? slots->mControllers.get() : nullptr;
+  }
+
+  bool SupportsAccessKey() const;
+  void RegUnRegAccessKey(bool aDoReg) override;
+  bool BoolAttrIsTrue(nsAtom* aName) const;
+
+  friend nsXULElement* NS_NewBasicXULElement(
+      already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo);
+
+  friend nsresult NS_NewXULElement(mozilla::dom::Element** aResult,
+                                   mozilla::dom::NodeInfo* aNodeInfo,
+                                   mozilla::dom::FromParser aFromParser,
+                                   const nsAString* aIs);
+  friend void NS_TrustedNewXULElement(mozilla::dom::Element** aResult,
+                                      mozilla::dom::NodeInfo* aNodeInfo);
+
+  static already_AddRefed<nsXULElement> CreateFromPrototype(
+      nsXULPrototypeElement* aPrototype, mozilla::dom::NodeInfo* aNodeInfo,
+      bool aIsScriptable, bool aIsRoot);
+
+  JSObject* WrapNode(JSContext*, JS::Handle<JSObject*> aGivenProto) override;
+
+  bool IsEventStoppedFromAnonymousScrollbar(mozilla::EventMessage aMessage);
+
+  MOZ_CAN_RUN_SCRIPT
+  nsresult DispatchXULCommand(const mozilla::EventChainVisitor& aVisitor,
+                              nsAutoString& aCommand);
+};
+
+#endif  // nsXULElement_h__
diff --git a/dom/xul/nsXULPopupListener.cpp b/dom/xul/nsXULPopupListener.cpp
new file mode 100644
index 0000000000..9f2ee88e8d
--- /dev/null
+++ b/dom/xul/nsXULPopupListener.cpp
@@ -0,0 +1,283 @@
+/* -*- 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/. */
+
+/*
+  This file provides the implementation for xul popup listener which
+  tracks xul popups and context menus
+ */
+
+#include "nsXULPopupListener.h"
+#include "XULButtonElement.h"
+#include "nsCOMPtr.h"
+#include "nsGkAtoms.h"
+#include "nsContentCID.h"
+#include "nsContentUtils.h"
+#include "nsXULPopupManager.h"
+#include "nsIScriptContext.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "nsServiceManagerUtils.h"
+#include "nsLayoutUtils.h"
+#include "mozilla/ReflowInput.h"
+#include "nsIObjectLoadingContent.h"
+#include "mozilla/BasePrincipal.h"
+#include "mozilla/EventStateManager.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/Event.h"  // for Event
+#include "mozilla/dom/EventTarget.h"
+#include "mozilla/dom/FragmentOrElement.h"
+#include "mozilla/dom/MouseEvent.h"
+#include "mozilla/dom/MouseEventBinding.h"
+
+// for event firing in context menus
+#include "nsPresContext.h"
+#include "nsFocusManager.h"
+#include "nsPIDOMWindow.h"
+#include "nsViewManager.h"
+#include "nsError.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+// on win32 and os/2, context menus come up on mouse up. On other platforms,
+// they appear on mouse down. Certain bits of code care about this difference.
+#if defined(XP_WIN)
+#  define NS_CONTEXT_MENU_IS_MOUSEUP 1
+#endif
+
+nsXULPopupListener::nsXULPopupListener(mozilla::dom::Element* aElement,
+                                       bool aIsContext)
+    : mElement(aElement), mPopupContent(nullptr), mIsContext(aIsContext) {}
+
+nsXULPopupListener::~nsXULPopupListener(void) { ClosePopup(); }
+
+NS_IMPL_CYCLE_COLLECTION(nsXULPopupListener, mElement, mPopupContent)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsXULPopupListener)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsXULPopupListener)
+
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_BEGIN(nsXULPopupListener)
+  // If the owner, mElement, can be skipped, so can we.
+  if (tmp->mElement) {
+    return mozilla::dom::FragmentOrElement::CanSkip(tmp->mElement, true);
+  }
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_END
+
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_IN_CC_BEGIN(nsXULPopupListener)
+  if (tmp->mElement) {
+    return mozilla::dom::FragmentOrElement::CanSkipInCC(tmp->mElement);
+  }
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_IN_CC_END
+
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_BEGIN(nsXULPopupListener)
+  if (tmp->mElement) {
+    return mozilla::dom::FragmentOrElement::CanSkipThis(tmp->mElement);
+  }
+NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsXULPopupListener)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMEventListener)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+////////////////////////////////////////////////////////////////
+// nsIDOMEventListener
+
+nsresult nsXULPopupListener::HandleEvent(Event* aEvent) {
+  nsAutoString eventType;
+  aEvent->GetType(eventType);
+
+  if (!((eventType.EqualsLiteral("mousedown") && !mIsContext) ||
+        (eventType.EqualsLiteral("contextmenu") && mIsContext)))
+    return NS_OK;
+
+  MouseEvent* mouseEvent = aEvent->AsMouseEvent();
+  if (!mouseEvent) {
+    // non-ui event passed in.  bad things.
+    return NS_OK;
+  }
+
+  // Get the node that was clicked on.
+  nsCOMPtr<nsIContent> targetContent =
+      nsIContent::FromEventTargetOrNull(mouseEvent->GetTarget());
+  if (!targetContent) {
+    return NS_OK;
+  }
+
+  if (nsIContent* content =
+          nsIContent::FromEventTargetOrNull(mouseEvent->GetOriginalTarget())) {
+    if (EventStateManager::IsTopLevelRemoteTarget(content)) {
+      return NS_OK;
+    }
+  }
+
+  bool preventDefault = mouseEvent->DefaultPrevented();
+  if (preventDefault && mIsContext) {
+    // Someone called preventDefault on a context menu.
+    // Let's make sure they are allowed to do so.
+    bool eventEnabled =
+        Preferences::GetBool("dom.event.contextmenu.enabled", true);
+    if (!eventEnabled) {
+      // The user wants his contextmenus.  Let's make sure that this is a
+      // website and not chrome since there could be places in chrome which
+      // don't want contextmenus.
+      if (!targetContent->NodePrincipal()->IsSystemPrincipal()) {
+        // This isn't chrome.  Cancel the preventDefault() and
+        // let the event go forth.
+        preventDefault = false;
+      }
+    }
+  }
+
+  if (preventDefault) {
+    // someone called preventDefault. bail.
+    return NS_OK;
+  }
+
+  // prevent popups on menu and menuitems as they handle their own popups
+  // This was added for bug 96920.
+  // If a menu item child was clicked on that leads to a popup needing
+  // to show, we know (guaranteed) that we're dealing with a menu or
+  // submenu of an already-showing popup.  We don't need to do anything at all.
+  if (!mIsContext &&
+      targetContent->IsAnyOfXULElements(nsGkAtoms::menu, nsGkAtoms::menuitem)) {
+    return NS_OK;
+  }
+
+  if (!mIsContext && mouseEvent->Button() != 0) {
+    // Only open popups when the left mouse button is down.
+    return NS_OK;
+  }
+
+  // Open the popup. LaunchPopup will call StopPropagation and PreventDefault
+  // in the right situations.
+  LaunchPopup(mouseEvent);
+
+  return NS_OK;
+}
+
+// ClosePopup
+//
+// Do everything needed to shut down the popup.
+//
+// NOTE: This routine is safe to call even if the popup is already closed.
+//
+void nsXULPopupListener::ClosePopup() {
+  if (mPopupContent) {
+    // this is called when the listener is going away, so make sure that the
+    // popup is hidden. Use asynchronous hiding just to be safe so we don't
+    // fire events during destruction.
+    nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+    if (pm)
+      pm->HidePopup(mPopupContent,
+                    {HidePopupOption::DeselectMenu, HidePopupOption::Async});
+    mPopupContent = nullptr;  // release the popup
+  }
+}  // ClosePopup
+
+static already_AddRefed<Element> GetImmediateChild(nsIContent* aContent,
+                                                   nsAtom* aTag) {
+  for (nsIContent* child = aContent->GetFirstChild(); child;
+       child = child->GetNextSibling()) {
+    if (child->IsXULElement(aTag)) {
+      RefPtr<Element> ret = child->AsElement();
+      return ret.forget();
+    }
+  }
+
+  return nullptr;
+}
+
+//
+// LaunchPopup
+//
+// Given the element on which the event was triggered and the mouse locations in
+// Client and widget coordinates, popup a new window showing the appropriate
+// content.
+//
+// aTargetContent is the target of the mouse event aEvent that triggered the
+// popup. mElement is the element that the popup menu is attached to.
+// aTargetContent may be equal to mElement or it may be a descendant.
+//
+// This looks for an attribute on |mElement| of the appropriate popup type
+// (popup, context) and uses that attribute's value as an ID for
+// the popup content in the document.
+//
+nsresult nsXULPopupListener::LaunchPopup(MouseEvent* aEvent) {
+  nsresult rv = NS_OK;
+
+  nsAutoString identifier;
+  nsAtom* type = mIsContext ? nsGkAtoms::context : nsGkAtoms::popup;
+  bool hasPopupAttr = mElement->GetAttr(type, identifier);
+
+  if (identifier.IsEmpty()) {
+    hasPopupAttr =
+        mElement->GetAttr(mIsContext ? nsGkAtoms::contextmenu : nsGkAtoms::menu,
+                          identifier) ||
+        hasPopupAttr;
+  }
+
+  if (hasPopupAttr) {
+    aEvent->StopPropagation();
+    aEvent->PreventDefault();
+  }
+
+  if (identifier.IsEmpty()) return rv;
+
+  // Try to find the popup content and the document.
+  nsCOMPtr<Document> document = mElement->GetComposedDoc();
+  if (!document) {
+    NS_WARNING("No document!");
+    return NS_ERROR_FAILURE;
+  }
+
+  // Handle the _child case for popups and context menus
+  RefPtr<Element> popup;
+  if (identifier.EqualsLiteral("_child")) {
+    popup = GetImmediateChild(mElement, nsGkAtoms::menupopup);
+  } else if (!mElement->IsInUncomposedDoc() ||
+             !(popup = document->GetElementById(identifier))) {
+    // XXXsmaug Should we try to use ShadowRoot::GetElementById in case
+    //          mElement is in shadow DOM?
+    //
+    // Use getElementById to obtain the popup content and gracefully fail if
+    // we didn't find any popup content in the document.
+    NS_WARNING("GetElementById had some kind of spasm.");
+    return rv;
+  }
+
+  // return if no popup was found or the popup is the element itself.
+  if (!popup || popup == mElement) {
+    return NS_OK;
+  }
+
+  // Submenus can't be used as context menus or popups, bug 288763.
+  // Similar code also in nsXULTooltipListener::GetTooltipFor.
+  if (auto* button = XULButtonElement::FromNodeOrNull(popup->GetParent())) {
+    if (button->IsMenu()) {
+      return NS_OK;
+    }
+  }
+
+  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
+  if (!pm) return NS_OK;
+
+  // For left-clicks, if the popup has an position attribute, or both the
+  // popupanchor and popupalign attributes are used, anchor the popup to the
+  // element, otherwise just open it at the screen position where the mouse
+  // was clicked. Context menus always open at the mouse position.
+  mPopupContent = popup;
+  if (!mIsContext && (mPopupContent->HasAttr(nsGkAtoms::position) ||
+                      (mPopupContent->HasAttr(nsGkAtoms::popupanchor) &&
+                       mPopupContent->HasAttr(nsGkAtoms::popupalign)))) {
+    pm->ShowPopup(mPopupContent, mElement, u""_ns, 0, 0, false, true, false,
+                  aEvent);
+  } else {
+    CSSIntPoint pos = aEvent->ScreenPoint(CallerType::System);
+    pm->ShowPopupAtScreen(mPopupContent, pos.x, pos.y, mIsContext, aEvent);
+  }
+
+  return NS_OK;
+}
diff --git a/dom/xul/nsXULPopupListener.h b/dom/xul/nsXULPopupListener.h
new file mode 100644
index 0000000000..ac5a631118
--- /dev/null
+++ b/dom/xul/nsXULPopupListener.h
@@ -0,0 +1,59 @@
+/* -*- 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/. */
+
+/**
+ * This is the popup listener implementation for popup menus and context menus.
+ */
+
+#ifndef nsXULPopupListener_h___
+#define nsXULPopupListener_h___
+
+#include "nsCOMPtr.h"
+
+#include "nsIDOMEventListener.h"
+#include "nsCycleCollectionParticipant.h"
+
+class nsIContent;
+
+namespace mozilla::dom {
+class Element;
+class MouseEvent;
+}  // namespace mozilla::dom
+
+class nsXULPopupListener : public nsIDOMEventListener {
+ public:
+  // aElement is the element that the popup is attached to. If aIsContext is
+  // false, the popup opens on left click on aElement or a descendant. If
+  // aIsContext is true, the popup is a context menu which opens on a
+  // context menu event.
+  nsXULPopupListener(mozilla::dom::Element* aElement, bool aIsContext);
+
+  // nsISupports
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SKIPPABLE_CLASS(nsXULPopupListener)
+  NS_DECL_NSIDOMEVENTLISTENER
+
+ protected:
+  virtual ~nsXULPopupListener(void);
+
+  // open the popup. aEvent is the event that triggered the popup such as
+  // a mouse click and aTargetContent is the target of this event.
+  virtual nsresult LaunchPopup(mozilla::dom::MouseEvent* aEvent);
+
+  // close the popup when the listener goes away
+  virtual void ClosePopup();
+
+ private:
+  // |mElement| is the node to which this listener is attached.
+  RefPtr<mozilla::dom::Element> mElement;
+
+  // The popup that is getting shown on top of mElement.
+  RefPtr<mozilla::dom::Element> mPopupContent;
+
+  // true if a context popup
+  bool mIsContext;
+};
+
+#endif  // nsXULPopupListener_h___
diff --git a/dom/xul/nsXULPrototypeCache.cpp b/dom/xul/nsXULPrototypeCache.cpp
new file mode 100644
index 0000000000..c70492b07c
--- /dev/null
+++ b/dom/xul/nsXULPrototypeCache.cpp
@@ -0,0 +1,500 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsXULPrototypeCache.h"
+
+#include "nsXULPrototypeDocument.h"
+#include "nsIURI.h"
+#include "nsNetUtil.h"
+
+#include "nsIFile.h"
+#include "nsIMemoryReporter.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIObserverService.h"
+#include "nsIStorageStream.h"
+
+#include "nsAppDirectoryServiceDefs.h"
+
+#include "js/experimental/JSStencil.h"
+#include "js/TracingAPI.h"
+
+#include "mozilla/StyleSheetInlines.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_nglayout.h"
+#include "mozilla/scache/StartupCache.h"
+#include "mozilla/scache/StartupCacheUtils.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/intl/LocaleService.h"
+
+using namespace mozilla;
+using namespace mozilla::scache;
+using mozilla::intl::LocaleService;
+
+static const char kXULCacheInfoKey[] = "nsXULPrototypeCache.startupCache";
+#define CACHE_PREFIX(aCompilationTarget) "xulcache/" aCompilationTarget
+
+static void DisableXULCacheChangedCallback(const char* aPref, void* aClosure) {
+  if (nsXULPrototypeCache* cache = nsXULPrototypeCache::GetInstance()) {
+    if (!cache->IsEnabled()) {
+      // AbortCaching() calls Flush() for us.
+      cache->AbortCaching();
+    }
+  }
+}
+
+//----------------------------------------------------------------------
+
+nsXULPrototypeCache* nsXULPrototypeCache::sInstance = nullptr;
+
+nsXULPrototypeCache::nsXULPrototypeCache() = default;
+
+NS_IMPL_ISUPPORTS(nsXULPrototypeCache, nsIObserver)
+
+/* static */
+nsXULPrototypeCache* nsXULPrototypeCache::GetInstance() {
+  if (!sInstance) {
+    NS_ADDREF(sInstance = new nsXULPrototypeCache());
+
+    Preferences::RegisterCallback(
+        DisableXULCacheChangedCallback,
+        nsDependentCString(
+            StaticPrefs::GetPrefName_nglayout_debug_disable_xul_cache()));
+
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    if (obsSvc) {
+      nsXULPrototypeCache* p = sInstance;
+      obsSvc->AddObserver(p, "chrome-flush-caches", false);
+      obsSvc->AddObserver(p, NS_XPCOM_SHUTDOWN_OBSERVER_ID, false);
+      obsSvc->AddObserver(p, "startupcache-invalidate", false);
+    }
+  }
+  return sInstance;
+}
+
+//----------------------------------------------------------------------
+
+NS_IMETHODIMP
+nsXULPrototypeCache::Observe(nsISupports* aSubject, const char* aTopic,
+                             const char16_t* aData) {
+  if (!strcmp(aTopic, "chrome-flush-caches") ||
+      !strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID)) {
+    Flush();
+  } else if (!strcmp(aTopic, "startupcache-invalidate")) {
+    AbortCaching();
+  } else {
+    NS_WARNING("Unexpected observer topic.");
+  }
+  return NS_OK;
+}
+
+nsXULPrototypeDocument* nsXULPrototypeCache::GetPrototype(nsIURI* aURI) {
+  if (!aURI) return nullptr;
+
+  nsCOMPtr<nsIURI> uriWithoutRef;
+  NS_GetURIWithoutRef(aURI, getter_AddRefs(uriWithoutRef));
+
+  nsXULPrototypeDocument* protoDoc = mPrototypeTable.GetWeak(uriWithoutRef);
+  if (protoDoc) {
+    return protoDoc;
+  }
+
+  nsresult rv = BeginCaching(aURI);
+  if (NS_FAILED(rv)) return nullptr;
+
+  // No prototype in XUL memory cache. Spin up the cache Service.
+  nsCOMPtr<nsIObjectInputStream> ois;
+  rv = GetPrototypeInputStream(aURI, getter_AddRefs(ois));
+  if (NS_FAILED(rv)) {
+    return nullptr;
+  }
+
+  RefPtr<nsXULPrototypeDocument> newProto;
+  rv = NS_NewXULPrototypeDocument(getter_AddRefs(newProto));
+  if (NS_FAILED(rv)) return nullptr;
+
+  rv = newProto->Read(ois);
+  if (NS_SUCCEEDED(rv)) {
+    rv = PutPrototype(newProto);
+  } else {
+    newProto = nullptr;
+  }
+
+  mInputStreamTable.Remove(aURI);
+  return newProto;
+}
+
+nsresult nsXULPrototypeCache::PutPrototype(nsXULPrototypeDocument* aDocument) {
+  if (!aDocument->GetURI()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIURI> uri;
+  NS_GetURIWithoutRef(aDocument->GetURI(), getter_AddRefs(uri));
+
+  // Put() releases any old value
+  mPrototypeTable.InsertOrUpdate(uri, RefPtr{aDocument});
+
+  return NS_OK;
+}
+
+JS::Stencil* nsXULPrototypeCache::GetStencil(nsIURI* aURI) {
+  if (auto* entry = mStencilTable.GetEntry(aURI)) {
+    return entry->mStencil;
+  }
+  return nullptr;
+}
+
+nsresult nsXULPrototypeCache::PutStencil(nsIURI* aURI, JS::Stencil* aStencil) {
+  MOZ_ASSERT(aStencil, "Need a non-NULL stencil");
+
+#ifdef DEBUG_BUG_392650
+  if (mStencilTable.Get(aURI)) {
+    nsAutoCString scriptName;
+    aURI->GetSpec(scriptName);
+    nsAutoCString message("Loaded script ");
+    message += scriptName;
+    message += " twice (bug 392650)";
+    NS_WARNING(message.get());
+  }
+#endif
+
+  mStencilTable.PutEntry(aURI)->mStencil = aStencil;
+
+  return NS_OK;
+}
+
+void nsXULPrototypeCache::Flush() {
+  mPrototypeTable.Clear();
+  mStencilTable.Clear();
+}
+
+bool nsXULPrototypeCache::IsEnabled() {
+  return !StaticPrefs::nglayout_debug_disable_xul_cache();
+}
+
+void nsXULPrototypeCache::AbortCaching() {
+  // Flush the XUL cache for good measure, in case we cached a bogus/downrev
+  // script, somehow.
+  Flush();
+
+  // Clear the cache set
+  mStartupCacheURITable.Clear();
+}
+
+nsresult nsXULPrototypeCache::WritePrototype(
+    nsXULPrototypeDocument* aPrototypeDocument) {
+  nsresult rv = NS_OK, rv2 = NS_OK;
+
+  if (!StartupCache::GetSingleton()) return NS_OK;
+
+  nsCOMPtr<nsIURI> protoURI = aPrototypeDocument->GetURI();
+
+  nsCOMPtr<nsIObjectOutputStream> oos;
+  rv = GetPrototypeOutputStream(protoURI, getter_AddRefs(oos));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = aPrototypeDocument->Write(oos);
+  NS_ENSURE_SUCCESS(rv, rv);
+  FinishPrototypeOutputStream(protoURI);
+  return NS_FAILED(rv) ? rv : rv2;
+}
+
+static nsresult PathifyURIForType(nsXULPrototypeCache::CacheType cacheType,
+                                  nsIURI* in, nsACString& out) {
+  switch (cacheType) {
+    case nsXULPrototypeCache::CacheType::Prototype:
+      return PathifyURI(CACHE_PREFIX("proto"), in, out);
+    case nsXULPrototypeCache::CacheType::Script:
+      return PathifyURI(CACHE_PREFIX("script"), in, out);
+  }
+  MOZ_ASSERT_UNREACHABLE("unknown cache type?");
+  return NS_ERROR_UNEXPECTED;
+}
+
+nsresult nsXULPrototypeCache::GetInputStream(CacheType cacheType, nsIURI* uri,
+                                             nsIObjectInputStream** stream) {
+  nsAutoCString spec;
+  nsresult rv = PathifyURIForType(cacheType, uri, spec);
+  if (NS_FAILED(rv)) return NS_ERROR_NOT_AVAILABLE;
+
+  const char* buf;
+  uint32_t len;
+  nsCOMPtr<nsIObjectInputStream> ois;
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) return NS_ERROR_NOT_AVAILABLE;
+
+  rv = sc->GetBuffer(spec.get(), &buf, &len);
+  if (NS_FAILED(rv)) return NS_ERROR_NOT_AVAILABLE;
+
+  rv = NewObjectInputStreamFromBuffer(buf, len, getter_AddRefs(ois));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  mInputStreamTable.InsertOrUpdate(uri, ois);
+
+  ois.forget(stream);
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeCache::FinishInputStream(nsIURI* uri) {
+  mInputStreamTable.Remove(uri);
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeCache::GetOutputStream(nsIURI* uri,
+                                              nsIObjectOutputStream** stream) {
+  nsresult rv;
+  nsCOMPtr<nsIObjectOutputStream> objectOutput;
+  nsCOMPtr<nsIStorageStream> storageStream;
+  bool found = mOutputStreamTable.Get(uri, getter_AddRefs(storageStream));
+  if (found) {
+    // Setting an output stream here causes crashes on Windows. The previous
+    // version of this code always returned NS_ERROR_OUT_OF_MEMORY here,
+    // because it used a mistyped contract ID to create its object stream.
+    return NS_ERROR_NOT_IMPLEMENTED;
+#if 0
+        nsCOMPtr<nsIOutputStream> outputStream
+            = do_QueryInterface(storageStream);
+        objectOutput = NS_NewObjectOutputStream(outputStream);
+#endif
+  } else {
+    rv = NewObjectOutputWrappedStorageStream(
+        getter_AddRefs(objectOutput), getter_AddRefs(storageStream), false);
+    NS_ENSURE_SUCCESS(rv, rv);
+    mOutputStreamTable.InsertOrUpdate(uri, storageStream);
+  }
+  objectOutput.forget(stream);
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeCache::FinishOutputStream(CacheType cacheType,
+                                                 nsIURI* uri) {
+  nsresult rv;
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) return NS_ERROR_NOT_AVAILABLE;
+
+  nsCOMPtr<nsIStorageStream> storageStream;
+  bool found = mOutputStreamTable.Get(uri, getter_AddRefs(storageStream));
+  if (!found) return NS_ERROR_UNEXPECTED;
+  nsCOMPtr<nsIOutputStream> outputStream = do_QueryInterface(storageStream);
+  outputStream->Close();
+
+  UniqueFreePtr<char[]> buf;
+  uint32_t len;
+  rv = NewBufferFromStorageStream(storageStream, &buf, &len);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (!mStartupCacheURITable.GetEntry(uri)) {
+    nsAutoCString spec;
+    rv = PathifyURIForType(cacheType, uri, spec);
+    if (NS_FAILED(rv)) return NS_ERROR_NOT_AVAILABLE;
+    rv = sc->PutBuffer(spec.get(), std::move(buf), len);
+    if (NS_SUCCEEDED(rv)) {
+      mOutputStreamTable.Remove(uri);
+      mStartupCacheURITable.PutEntry(uri);
+    }
+  }
+
+  return rv;
+}
+
+// We have data if we're in the middle of writing it or we already
+// have it in the cache.
+nsresult nsXULPrototypeCache::HasData(CacheType cacheType, nsIURI* uri,
+                                      bool* exists) {
+  if (mOutputStreamTable.Get(uri, nullptr)) {
+    *exists = true;
+    return NS_OK;
+  }
+  nsAutoCString spec;
+  nsresult rv = PathifyURIForType(cacheType, uri, spec);
+  if (NS_FAILED(rv)) {
+    *exists = false;
+    return NS_OK;
+  }
+  UniquePtr<char[]> buf;
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (sc) {
+    *exists = sc->HasEntry(spec.get());
+  } else {
+    *exists = false;
+  }
+  return NS_OK;
+}
+
+nsresult nsXULPrototypeCache::BeginCaching(nsIURI* aURI) {
+  nsresult rv, tmp;
+
+  nsAutoCString path;
+  aURI->GetPathQueryRef(path);
+  if (!(StringEndsWith(path, ".xul"_ns) || StringEndsWith(path, ".xhtml"_ns))) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  StartupCache* startupCache = StartupCache::GetSingleton();
+  if (!startupCache) return NS_ERROR_FAILURE;
+
+  if (StaticPrefs::nglayout_debug_disable_xul_cache()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  // Get the chrome directory to validate against the one stored in the
+  // cache file, or to store there if we're generating a new file.
+  nsCOMPtr<nsIFile> chromeDir;
+  rv = NS_GetSpecialDirectory(NS_APP_CHROME_DIR, getter_AddRefs(chromeDir));
+  if (NS_FAILED(rv)) return rv;
+  nsAutoCString chromePath;
+  rv = chromeDir->GetPersistentDescriptor(chromePath);
+  if (NS_FAILED(rv)) return rv;
+
+  // XXXbe we assume the first package's locale is the same as the locale of
+  // all subsequent packages of cached chrome URIs....
+  nsAutoCString package;
+  rv = aURI->GetHost(package);
+  if (NS_FAILED(rv)) return rv;
+  nsAutoCString locale;
+  LocaleService::GetInstance()->GetAppLocaleAsBCP47(locale);
+
+  nsAutoCString fileChromePath, fileLocale;
+
+  const char* buf = nullptr;
+  uint32_t len, amtRead;
+  nsCOMPtr<nsIObjectInputStream> objectInput;
+
+  rv = startupCache->GetBuffer(kXULCacheInfoKey, &buf, &len);
+  if (NS_SUCCEEDED(rv))
+    rv = NewObjectInputStreamFromBuffer(buf, len, getter_AddRefs(objectInput));
+
+  if (NS_SUCCEEDED(rv)) {
+    rv = objectInput->ReadCString(fileLocale);
+    tmp = objectInput->ReadCString(fileChromePath);
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+    if (NS_FAILED(rv) ||
+        (!fileChromePath.Equals(chromePath) || !fileLocale.Equals(locale))) {
+      // Our cache won't be valid in this case, we'll need to rewrite.
+      // XXX This blows away work that other consumers (like
+      // mozJSModuleLoader) have done, need more fine-grained control.
+      startupCache->InvalidateCache();
+      mStartupCacheURITable.Clear();
+      rv = NS_ERROR_UNEXPECTED;
+    }
+  } else if (rv != NS_ERROR_NOT_AVAILABLE)
+    // NS_ERROR_NOT_AVAILABLE is normal, usually if there's no cachefile.
+    return rv;
+
+  if (NS_FAILED(rv)) {
+    // Either the cache entry was invalid or it didn't exist, so write it now.
+    nsCOMPtr<nsIObjectOutputStream> objectOutput;
+    nsCOMPtr<nsIInputStream> inputStream;
+    nsCOMPtr<nsIStorageStream> storageStream;
+    rv = NewObjectOutputWrappedStorageStream(
+        getter_AddRefs(objectOutput), getter_AddRefs(storageStream), false);
+    if (NS_SUCCEEDED(rv)) {
+      rv = objectOutput->WriteStringZ(locale.get());
+      tmp = objectOutput->WriteStringZ(chromePath.get());
+      if (NS_FAILED(tmp)) {
+        rv = tmp;
+      }
+      tmp = objectOutput->Close();
+      if (NS_FAILED(tmp)) {
+        rv = tmp;
+      }
+      tmp = storageStream->NewInputStream(0, getter_AddRefs(inputStream));
+      if (NS_FAILED(tmp)) {
+        rv = tmp;
+      }
+    }
+
+    if (NS_SUCCEEDED(rv)) {
+      uint64_t len64;
+      rv = inputStream->Available(&len64);
+      if (NS_SUCCEEDED(rv)) {
+        if (len64 <= UINT32_MAX)
+          len = (uint32_t)len64;
+        else
+          rv = NS_ERROR_FILE_TOO_BIG;
+      }
+    }
+
+    if (NS_SUCCEEDED(rv)) {
+      auto putBuf = UniqueFreePtr<char[]>(
+          reinterpret_cast<char*>(malloc(sizeof(char) * len)));
+      rv = inputStream->Read(putBuf.get(), len, &amtRead);
+      if (NS_SUCCEEDED(rv) && len == amtRead)
+        rv = startupCache->PutBuffer(kXULCacheInfoKey, std::move(putBuf), len);
+      else {
+        rv = NS_ERROR_UNEXPECTED;
+      }
+    }
+
+    // Failed again, just bail.
+    if (NS_FAILED(rv)) {
+      startupCache->InvalidateCache();
+      mStartupCacheURITable.Clear();
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  return NS_OK;
+}
+
+void nsXULPrototypeCache::MarkInCCGeneration(uint32_t aGeneration) {
+  for (const auto& prototype : mPrototypeTable.Values()) {
+    prototype->MarkInCCGeneration(aGeneration);
+  }
+}
+
+MOZ_DEFINE_MALLOC_SIZE_OF(CacheMallocSizeOf)
+
+static void ReportSize(const nsCString& aPath, size_t aAmount,
+                       const nsCString& aDescription,
+                       nsIHandleReportCallback* aHandleReport,
+                       nsISupports* aData) {
+  nsAutoCString path("explicit/xul-prototype-cache/");
+  path += aPath;
+  aHandleReport->Callback(""_ns, path, nsIMemoryReporter::KIND_HEAP,
+                          nsIMemoryReporter::UNITS_BYTES, aAmount, aDescription,
+                          aData);
+}
+
+/* static */
+void nsXULPrototypeCache::CollectMemoryReports(
+    nsIHandleReportCallback* aHandleReport, nsISupports* aData) {
+  if (!sInstance) {
+    return;
+  }
+
+  MallocSizeOf mallocSizeOf = CacheMallocSizeOf;
+  size_t other = mallocSizeOf(sInstance);
+
+#define REPORT_SIZE(_path, _amount, _desc) \
+  ReportSize(_path, _amount, nsLiteralCString(_desc), aHandleReport, aData)
+
+  other += sInstance->mPrototypeTable.ShallowSizeOfExcludingThis(mallocSizeOf);
+  // TODO Report content in mPrototypeTable?
+
+  other += sInstance->mStencilTable.ShallowSizeOfExcludingThis(mallocSizeOf);
+  // TODO Report content inside mStencilTable?
+
+  other +=
+      sInstance->mStartupCacheURITable.ShallowSizeOfExcludingThis(mallocSizeOf);
+
+  other +=
+      sInstance->mOutputStreamTable.ShallowSizeOfExcludingThis(mallocSizeOf);
+  other +=
+      sInstance->mInputStreamTable.ShallowSizeOfExcludingThis(mallocSizeOf);
+
+  REPORT_SIZE("other"_ns, other,
+              "Memory used by "
+              "the instance and tables of the XUL prototype cache.");
+
+#undef REPORT_SIZE
+}
diff --git a/dom/xul/nsXULPrototypeCache.h b/dom/xul/nsXULPrototypeCache.h
new file mode 100644
index 0000000000..657bf92511
--- /dev/null
+++ b/dom/xul/nsXULPrototypeCache.h
@@ -0,0 +1,164 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+#ifndef nsXULPrototypeCache_h__
+#define nsXULPrototypeCache_h__
+
+#include "nsBaseHashtable.h"
+#include "nsCOMPtr.h"
+#include "nsIObserver.h"
+#include "nsInterfaceHashtable.h"
+#include "nsRefPtrHashtable.h"
+#include "nsURIHashKey.h"
+#include "nsXULPrototypeDocument.h"
+#include "nsIStorageStream.h"
+
+#include "mozilla/scache/StartupCache.h"
+#include "js/experimental/JSStencil.h"
+#include "mozilla/RefPtr.h"
+
+class nsIHandleReportCallback;
+namespace mozilla {
+class StyleSheet;
+}  // namespace mozilla
+
+/**
+ * The XUL prototype cache can be used to store and retrieve shared data for
+ * XUL documents, style sheets, XBL, and scripts.
+ *
+ * The cache has two levels:
+ *  1. In-memory hashtables
+ *  2. The on-disk cache file.
+ */
+class nsXULPrototypeCache : public nsIObserver {
+ public:
+  enum class CacheType { Prototype, Script };
+
+  // nsISupports
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+
+  bool IsCached(nsIURI* aURI) { return GetPrototype(aURI) != nullptr; }
+  void AbortCaching();
+
+  /**
+   * Whether the prototype cache is enabled.
+   */
+  bool IsEnabled();
+
+  /**
+   * Flush the cache; remove all XUL prototype documents, style
+   * sheets, and scripts.
+   */
+  void Flush();
+
+  // The following methods are used to put and retrive various items into and
+  // from the cache.
+
+  nsXULPrototypeDocument* GetPrototype(nsIURI* aURI);
+  nsresult PutPrototype(nsXULPrototypeDocument* aDocument);
+  void RemovePrototype(nsIURI* aURI) { mPrototypeTable.Remove(aURI); }
+
+  JS::Stencil* GetStencil(nsIURI* aURI);
+  nsresult PutStencil(nsIURI* aURI, JS::Stencil* aStencil);
+
+  /**
+   * Write the XUL prototype document to a cache file. The proto must be
+   * fully loaded.
+   */
+  nsresult WritePrototype(nsXULPrototypeDocument* aPrototypeDocument);
+
+  /**
+   * This interface allows partial reads and writes from the buffers in the
+   * startupCache.
+   */
+
+  inline nsresult GetPrototypeInputStream(nsIURI* aURI,
+                                          nsIObjectInputStream** objectInput) {
+    return GetInputStream(CacheType::Prototype, aURI, objectInput);
+  }
+  inline nsresult GetScriptInputStream(nsIURI* aURI,
+                                       nsIObjectInputStream** objectInput) {
+    return GetInputStream(CacheType::Script, aURI, objectInput);
+  }
+  inline nsresult FinishScriptInputStream(nsIURI* aURI) {
+    return FinishInputStream(aURI);
+  }
+
+  inline nsresult GetPrototypeOutputStream(
+      nsIURI* aURI, nsIObjectOutputStream** objectOutput) {
+    return GetOutputStream(aURI, objectOutput);
+  }
+  inline nsresult GetScriptOutputStream(nsIURI* aURI,
+                                        nsIObjectOutputStream** objectOutput) {
+    return GetOutputStream(aURI, objectOutput);
+  }
+
+  inline nsresult FinishPrototypeOutputStream(nsIURI* aURI) {
+    return FinishOutputStream(CacheType::Prototype, aURI);
+  }
+  inline nsresult FinishScriptOutputStream(nsIURI* aURI) {
+    return FinishOutputStream(CacheType::Script, aURI);
+  }
+
+  inline nsresult HasPrototype(nsIURI* aURI, bool* exists) {
+    return HasData(CacheType::Prototype, aURI, exists);
+  }
+  inline nsresult HasScript(nsIURI* aURI, bool* exists) {
+    return HasData(CacheType::Script, aURI, exists);
+  }
+
+ private:
+  nsresult GetInputStream(CacheType cacheType, nsIURI* uri,
+                          nsIObjectInputStream** stream);
+  nsresult FinishInputStream(nsIURI* aURI);
+
+  nsresult GetOutputStream(nsIURI* aURI, nsIObjectOutputStream** objectOutput);
+  nsresult FinishOutputStream(CacheType cacheType, nsIURI* aURI);
+  nsresult HasData(CacheType cacheType, nsIURI* aURI, bool* exists);
+
+ public:
+  static nsXULPrototypeCache* GetInstance();
+  static nsXULPrototypeCache* MaybeGetInstance() { return sInstance; }
+
+  static void ReleaseGlobals() { NS_IF_RELEASE(sInstance); }
+
+  void MarkInCCGeneration(uint32_t aGeneration);
+
+  static void CollectMemoryReports(nsIHandleReportCallback* aHandleReport,
+                                   nsISupports* aData);
+
+ protected:
+  friend nsresult NS_NewXULPrototypeCache(REFNSIID aIID, void** aResult);
+
+  nsXULPrototypeCache();
+  virtual ~nsXULPrototypeCache() = default;
+
+  static nsXULPrototypeCache* sInstance;
+
+  nsRefPtrHashtable<nsURIHashKey, nsXULPrototypeDocument>
+      mPrototypeTable;  // owns the prototypes
+
+  class StencilHashKey : public nsURIHashKey {
+   public:
+    explicit StencilHashKey(const nsIURI* aKey) : nsURIHashKey(aKey) {}
+    StencilHashKey(StencilHashKey&&) = default;
+
+    RefPtr<JS::Stencil> mStencil;
+  };
+
+  nsTHashtable<StencilHashKey> mStencilTable;
+
+  // URIs already written to the startup cache, to prevent double-caching.
+  nsTHashtable<nsURIHashKey> mStartupCacheURITable;
+
+  nsInterfaceHashtable<nsURIHashKey, nsIStorageStream> mOutputStreamTable;
+  nsInterfaceHashtable<nsURIHashKey, nsIObjectInputStream> mInputStreamTable;
+
+  // Bootstrap caching service
+  nsresult BeginCaching(nsIURI* aDocumentURI);
+};
+
+#endif  // nsXULPrototypeCache_h__
diff --git a/dom/xul/nsXULPrototypeDocument.cpp b/dom/xul/nsXULPrototypeDocument.cpp
new file mode 100644
index 0000000000..d4f2b7962b
--- /dev/null
+++ b/dom/xul/nsXULPrototypeDocument.cpp
@@ -0,0 +1,511 @@
+/* -*- 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 "nsXULPrototypeDocument.h"
+
+#include "nsXULElement.h"
+#include "nsAString.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIPrincipal.h"
+#include "nsJSPrincipals.h"
+#include "nsIScriptObjectPrincipal.h"
+#include "nsIURI.h"
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "nsString.h"
+#include "nsDOMCID.h"
+#include "nsNodeInfoManager.h"
+#include "nsContentUtils.h"
+#include "nsCCUncollectableMarker.h"
+#include "xpcpublic.h"
+#include "mozilla/BasePrincipal.h"
+#include "mozilla/dom/BindingUtils.h"
+#include "nsXULPrototypeCache.h"
+#include "mozilla/DeclarationBlock.h"
+#include "mozilla/dom/CustomElementRegistry.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/Text.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+using mozilla::dom::DestroyProtoAndIfaceCache;
+
+uint32_t nsXULPrototypeDocument::gRefCnt;
+
+//----------------------------------------------------------------------
+//
+// ctors, dtors, n' stuff
+//
+
+nsXULPrototypeDocument::nsXULPrototypeDocument()
+    : mRoot(nullptr), mLoaded(false), mCCGeneration(0), mWasL10nCached(false) {
+  ++gRefCnt;
+}
+
+nsresult nsXULPrototypeDocument::Init() {
+  mNodeInfoManager = new nsNodeInfoManager(nullptr, nullptr);
+  return NS_OK;
+}
+
+nsXULPrototypeDocument::~nsXULPrototypeDocument() {
+  if (mRoot) mRoot->ReleaseSubtree();
+}
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsXULPrototypeDocument)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsXULPrototypeDocument)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mPrototypeWaiters)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsXULPrototypeDocument)
+  if (nsCCUncollectableMarker::InGeneration(cb, tmp->mCCGeneration)) {
+    return NS_SUCCESS_INTERRUPTED_TRAVERSE;
+  }
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mRoot)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mNodeInfoManager)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsXULPrototypeDocument)
+  NS_INTERFACE_MAP_ENTRY(nsISerializable)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsXULPrototypeDocument)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsXULPrototypeDocument)
+
+NS_IMETHODIMP
+NS_NewXULPrototypeDocument(nsXULPrototypeDocument** aResult) {
+  *aResult = nullptr;
+  RefPtr<nsXULPrototypeDocument> doc = new nsXULPrototypeDocument();
+
+  nsresult rv = doc->Init();
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  doc.forget(aResult);
+  return rv;
+}
+
+//----------------------------------------------------------------------
+//
+// nsISerializable methods
+//
+
+NS_IMETHODIMP
+nsXULPrototypeDocument::Read(nsIObjectInputStream* aStream) {
+  nsCOMPtr<nsISupports> supports;
+  nsresult rv = aStream->ReadObject(true, getter_AddRefs(supports));
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  mURI = do_QueryInterface(supports);
+
+  // nsIPrincipal mNodeInfoManager->mPrincipal
+  nsAutoCString JSON;
+  rv = aStream->ReadCString(JSON);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  nsCOMPtr<nsIPrincipal> principal = mozilla::BasePrincipal::FromJSON(JSON);
+
+  // Better safe than sorry....
+  mNodeInfoManager->SetDocumentPrincipal(principal);
+
+  rv = aStream->ReadBoolean(&mWasL10nCached);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  mRoot = new nsXULPrototypeElement();
+
+  // mozilla::dom::NodeInfo table
+  nsTArray<RefPtr<mozilla::dom::NodeInfo>> nodeInfos;
+
+  uint32_t count, i;
+  rv = aStream->Read32(&count);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  nsAutoString namespaceURI, prefixStr, localName;
+  bool prefixIsNull;
+  RefPtr<nsAtom> prefix;
+  for (i = 0; i < count; ++i) {
+    rv = aStream->ReadString(namespaceURI);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+    rv = aStream->ReadBoolean(&prefixIsNull);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+    if (prefixIsNull) {
+      prefix = nullptr;
+    } else {
+      rv = aStream->ReadString(prefixStr);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      prefix = NS_Atomize(prefixStr);
+    }
+    rv = aStream->ReadString(localName);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+
+    RefPtr<mozilla::dom::NodeInfo> nodeInfo;
+    // Using UINT16_MAX here as we don't know which nodeinfos will be
+    // used for attributes and which for elements. And that doesn't really
+    // matter.
+    rv = mNodeInfoManager->GetNodeInfo(localName, prefix, namespaceURI,
+                                       UINT16_MAX, getter_AddRefs(nodeInfo));
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+    nodeInfos.AppendElement(nodeInfo);
+  }
+
+  // Document contents
+  uint32_t type;
+  while (NS_SUCCEEDED(rv)) {
+    rv = aStream->Read32(&type);
+    if (NS_FAILED(rv)) {
+      return rv;
+      break;
+    }
+
+    if ((nsXULPrototypeNode::Type)type == nsXULPrototypeNode::eType_PI) {
+      RefPtr<nsXULPrototypePI> pi = new nsXULPrototypePI();
+
+      rv = pi->Deserialize(aStream, this, mURI, &nodeInfos);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      rv = AddProcessingInstruction(pi);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+    } else if ((nsXULPrototypeNode::Type)type ==
+               nsXULPrototypeNode::eType_Element) {
+      rv = mRoot->Deserialize(aStream, this, mURI, &nodeInfos);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      break;
+    } else {
+      MOZ_ASSERT_UNREACHABLE("Unexpected prototype node type");
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  return NotifyLoadDone();
+}
+
+static nsresult GetNodeInfos(nsXULPrototypeElement* aPrototype,
+                             nsTArray<RefPtr<mozilla::dom::NodeInfo>>& aArray) {
+  if (aArray.IndexOf(aPrototype->mNodeInfo) == aArray.NoIndex) {
+    aArray.AppendElement(aPrototype->mNodeInfo);
+  }
+
+  // Search attributes
+  size_t i;
+  for (i = 0; i < aPrototype->mAttributes.Length(); ++i) {
+    RefPtr<mozilla::dom::NodeInfo> ni;
+    nsAttrName* name = &aPrototype->mAttributes[i].mName;
+    if (name->IsAtom()) {
+      ni = aPrototype->mNodeInfo->NodeInfoManager()->GetNodeInfo(
+          name->Atom(), nullptr, kNameSpaceID_None, nsINode::ATTRIBUTE_NODE);
+    } else {
+      ni = name->NodeInfo();
+    }
+
+    if (aArray.IndexOf(ni) == aArray.NoIndex) {
+      aArray.AppendElement(ni);
+    }
+  }
+
+  // Search children
+  for (i = 0; i < aPrototype->mChildren.Length(); ++i) {
+    nsXULPrototypeNode* child = aPrototype->mChildren[i];
+    if (child->mType == nsXULPrototypeNode::eType_Element) {
+      nsresult rv =
+          GetNodeInfos(static_cast<nsXULPrototypeElement*>(child), aArray);
+      NS_ENSURE_SUCCESS(rv, rv);
+    }
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULPrototypeDocument::Write(nsIObjectOutputStream* aStream) {
+  nsresult rv;
+
+  rv = aStream->WriteCompoundObject(mURI, NS_GET_IID(nsIURI), true);
+
+  // nsIPrincipal mNodeInfoManager->mPrincipal
+  nsAutoCString JSON;
+  mozilla::BasePrincipal::Cast(mNodeInfoManager->DocumentPrincipal())
+      ->ToJSON(JSON);
+  nsresult tmp = aStream->WriteStringZ(JSON.get());
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+#ifdef DEBUG
+  // XXX Worrisome if we're caching things without system principal.
+  if (!mNodeInfoManager->DocumentPrincipal()->IsSystemPrincipal()) {
+    NS_WARNING("Serializing document without system principal");
+  }
+#endif
+
+  tmp = aStream->WriteBoolean(mWasL10nCached);
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+
+  // mozilla::dom::NodeInfo table
+  nsTArray<RefPtr<mozilla::dom::NodeInfo>> nodeInfos;
+  if (mRoot) {
+    tmp = GetNodeInfos(mRoot, nodeInfos);
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+  }
+
+  uint32_t nodeInfoCount = nodeInfos.Length();
+  tmp = aStream->Write32(nodeInfoCount);
+  if (NS_FAILED(tmp)) {
+    rv = tmp;
+  }
+  uint32_t i;
+  for (i = 0; i < nodeInfoCount; ++i) {
+    mozilla::dom::NodeInfo* nodeInfo = nodeInfos[i];
+    NS_ENSURE_TRUE(nodeInfo, NS_ERROR_FAILURE);
+
+    nsAutoString namespaceURI;
+    nodeInfo->GetNamespaceURI(namespaceURI);
+    tmp = aStream->WriteWStringZ(namespaceURI.get());
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+
+    nsAutoString prefix;
+    nodeInfo->GetPrefix(prefix);
+    bool nullPrefix = DOMStringIsNull(prefix);
+    tmp = aStream->WriteBoolean(nullPrefix);
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+    if (!nullPrefix) {
+      tmp = aStream->WriteWStringZ(prefix.get());
+      if (NS_FAILED(tmp)) {
+        rv = tmp;
+      }
+    }
+
+    nsAutoString localName;
+    nodeInfo->GetName(localName);
+    tmp = aStream->WriteWStringZ(localName.get());
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+  }
+
+  // Now serialize the document contents
+  uint32_t count = mProcessingInstructions.Length();
+  for (i = 0; i < count; ++i) {
+    nsXULPrototypePI* pi = mProcessingInstructions[i];
+    tmp = pi->Serialize(aStream, this, &nodeInfos);
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+  }
+
+  if (mRoot) {
+    tmp = mRoot->Serialize(aStream, this, &nodeInfos);
+    if (NS_FAILED(tmp)) {
+      rv = tmp;
+    }
+  }
+
+  return rv;
+}
+
+//----------------------------------------------------------------------
+//
+
+nsresult nsXULPrototypeDocument::InitPrincipal(nsIURI* aURI,
+                                               nsIPrincipal* aPrincipal) {
+  NS_ENSURE_ARG_POINTER(aURI);
+
+  mURI = aURI;
+  mNodeInfoManager->SetDocumentPrincipal(aPrincipal);
+  return NS_OK;
+}
+
+nsIURI* nsXULPrototypeDocument::GetURI() {
+  NS_ASSERTION(mURI, "null URI");
+  return mURI;
+}
+
+nsXULPrototypeElement* nsXULPrototypeDocument::GetRootElement() {
+  return mRoot;
+}
+
+void nsXULPrototypeDocument::SetRootElement(nsXULPrototypeElement* aElement) {
+  mRoot = aElement;
+}
+
+nsresult nsXULPrototypeDocument::AddProcessingInstruction(
+    nsXULPrototypePI* aPI) {
+  MOZ_ASSERT(aPI, "null ptr");
+  // XXX(Bug 1631371) Check if this should use a fallible operation as it
+  // pretended earlier, or change the return type to void.
+  mProcessingInstructions.AppendElement(aPI);
+  return NS_OK;
+}
+
+const nsTArray<RefPtr<nsXULPrototypePI>>&
+nsXULPrototypeDocument::GetProcessingInstructions() const {
+  return mProcessingInstructions;
+}
+
+nsIPrincipal* nsXULPrototypeDocument::DocumentPrincipal() {
+  MOZ_ASSERT(mNodeInfoManager, "missing nodeInfoManager");
+  return mNodeInfoManager->DocumentPrincipal();
+}
+
+void nsXULPrototypeDocument::SetDocumentPrincipal(nsIPrincipal* aPrincipal) {
+  mNodeInfoManager->SetDocumentPrincipal(aPrincipal);
+}
+
+void nsXULPrototypeDocument::MarkInCCGeneration(uint32_t aCCGeneration) {
+  mCCGeneration = aCCGeneration;
+}
+
+nsNodeInfoManager* nsXULPrototypeDocument::GetNodeInfoManager() {
+  return mNodeInfoManager;
+}
+
+nsresult nsXULPrototypeDocument::AwaitLoadDone(Callback&& aCallback,
+                                               bool* aResult) {
+  nsresult rv = NS_OK;
+
+  *aResult = mLoaded;
+
+  if (!mLoaded) {
+    // XXX(Bug 1631371) Check if this should use a fallible operation as it
+    // pretended earlier, or change the return type to void.
+    mPrototypeWaiters.AppendElement(std::move(aCallback));
+  }
+
+  return rv;
+}
+
+nsresult nsXULPrototypeDocument::NotifyLoadDone() {
+  // Call back to each XUL document that raced to start the same
+  // prototype document load, lost the race, but hit the XUL
+  // prototype cache because the winner filled the cache with
+  // the not-yet-loaded prototype object.
+
+  mLoaded = true;
+
+  for (uint32_t i = mPrototypeWaiters.Length(); i > 0;) {
+    --i;
+    mPrototypeWaiters[i]();
+  }
+  mPrototypeWaiters.Clear();
+
+  return NS_OK;
+}
+
+void nsXULPrototypeDocument::SetIsL10nCached(bool aIsCached) {
+  mWasL10nCached = aIsCached;
+}
+
+void nsXULPrototypeDocument::RebuildPrototypeFromElement(
+    nsXULPrototypeElement* aPrototype, Element* aElement, bool aDeep) {
+  aPrototype->mHasIdAttribute = aElement->HasID();
+  aPrototype->mHasClassAttribute = aElement->MayHaveClass();
+  aPrototype->mHasStyleAttribute = aElement->MayHaveStyle();
+  NodeInfo* oldNodeInfo = aElement->NodeInfo();
+  RefPtr<NodeInfo> newNodeInfo = mNodeInfoManager->GetNodeInfo(
+      oldNodeInfo->NameAtom(), oldNodeInfo->GetPrefixAtom(),
+      oldNodeInfo->NamespaceID(), nsINode::ELEMENT_NODE);
+  aPrototype->mNodeInfo = newNodeInfo;
+
+  // First replace the prototype attributes with the new ones from this element.
+  aPrototype->mAttributes.Clear();
+
+  uint32_t count = aElement->GetAttrCount();
+  nsXULPrototypeAttribute* protoAttr =
+      aPrototype->mAttributes.AppendElements(count);
+  for (uint32_t index = 0; index < count; index++) {
+    BorrowedAttrInfo attr = aElement->GetAttrInfoAt(index);
+
+    if (attr.mName->IsAtom()) {
+      protoAttr->mName.SetTo(attr.mName->Atom());
+    } else {
+      NodeInfo* oldNodeInfo = attr.mName->NodeInfo();
+      RefPtr<NodeInfo> newNodeInfo = mNodeInfoManager->GetNodeInfo(
+          oldNodeInfo->NameAtom(), oldNodeInfo->GetPrefixAtom(),
+          oldNodeInfo->NamespaceID(), nsINode::ATTRIBUTE_NODE);
+      protoAttr->mName.SetTo(newNodeInfo);
+    }
+    protoAttr->mValue.SetTo(*attr.mValue);
+
+    protoAttr++;
+  }
+
+  // Make sure the mIsAtom is correct in case this prototype element has been
+  // completely rebuilt.
+  CustomElementData* ceData = aElement->GetCustomElementData();
+  nsAtom* isAtom = ceData ? ceData->GetIs(aElement) : nullptr;
+  aPrototype->mIsAtom = isAtom;
+
+  if (aDeep) {
+    // We have to rebuild the prototype children from this element.
+    // First release the tree under this element.
+    aPrototype->ReleaseSubtree();
+
+    RefPtr<nsXULPrototypeNode>* children =
+        aPrototype->mChildren.AppendElements(aElement->GetChildCount());
+    for (nsIContent* child = aElement->GetFirstChild(); child;
+         child = child->GetNextSibling()) {
+      if (child->IsElement()) {
+        Element* element = child->AsElement();
+        RefPtr<nsXULPrototypeElement> elemProto = new nsXULPrototypeElement;
+        RebuildPrototypeFromElement(elemProto, element, true);
+        *children = elemProto;
+      } else if (child->IsText()) {
+        Text* text = child->AsText();
+        RefPtr<nsXULPrototypeText> textProto = new nsXULPrototypeText();
+        text->AppendTextTo(textProto->mValue);
+        *children = textProto;
+      } else {
+        MOZ_ASSERT(false, "We handle only elements and text nodes here.");
+      }
+
+      children++;
+    }
+  }
+}
+
+void nsXULPrototypeDocument::RebuildL10nPrototype(Element* aElement,
+                                                  bool aDeep) {
+  if (mWasL10nCached) {
+    return;
+  }
+
+  MOZ_ASSERT(aElement->HasAttr(nsGkAtoms::datal10nid));
+
+  Document* doc = aElement->OwnerDoc();
+  if (RefPtr<nsXULPrototypeElement> proto =
+          doc->mL10nProtoElements.Get(aElement)) {
+    RebuildPrototypeFromElement(proto, aElement, aDeep);
+  }
+}
diff --git a/dom/xul/nsXULPrototypeDocument.h b/dom/xul/nsXULPrototypeDocument.h
new file mode 100644
index 0000000000..511f4b3e2b
--- /dev/null
+++ b/dom/xul/nsXULPrototypeDocument.h
@@ -0,0 +1,127 @@
+/* -*- Mode: C++; tab-width: 4; 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/. */
+
+#ifndef nsXULPrototypeDocument_h__
+#define nsXULPrototypeDocument_h__
+
+#include "js/TracingAPI.h"
+#include "mozilla/Attributes.h"
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+#include "nsISerializable.h"
+#include "nsCycleCollectionParticipant.h"
+#include <functional>
+
+class nsAtom;
+class nsIPrincipal;
+class nsIURI;
+class nsNodeInfoManager;
+class nsXULPrototypeElement;
+class nsXULPrototypePI;
+
+namespace mozilla::dom {
+class Element;
+}
+
+/**
+ * A "prototype" document that stores shared document information
+ * for the XUL cache.
+ * Among other things, stores the tree of nsXULPrototype*
+ * objects, from which the real DOM tree is built later in
+ * PrototypeDocumentContentSink::ResumeWalk.
+ */
+class nsXULPrototypeDocument final : public nsISerializable {
+ public:
+  using Callback = std::function<void()>;
+
+  // nsISupports interface
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+
+  // nsISerializable interface
+  NS_DECL_NSISERIALIZABLE
+
+  nsresult InitPrincipal(nsIURI* aURI, nsIPrincipal* aPrincipal);
+  nsIURI* GetURI();
+
+  /**
+   * Get/set the root nsXULPrototypeElement of the document.
+   */
+  nsXULPrototypeElement* GetRootElement();
+  void SetRootElement(nsXULPrototypeElement* aElement);
+
+  /**
+   * Add a processing instruction to the prolog. Note that only
+   * PI nodes are currently stored in a XUL prototype document's
+   * prolog and that they're handled separately from the rest of
+   * prototype node tree.
+   *
+   * @param aPI an already adrefed PI proto to add. This method takes
+   *            ownership of the passed PI.
+   */
+  nsresult AddProcessingInstruction(nsXULPrototypePI* aPI);
+  /**
+   * @note GetProcessingInstructions retains the ownership (the PI
+   *       protos only get deleted when the proto document is deleted)
+   */
+  const nsTArray<RefPtr<nsXULPrototypePI> >& GetProcessingInstructions() const;
+
+  nsIPrincipal* DocumentPrincipal();
+  void SetDocumentPrincipal(nsIPrincipal* aPrincipal);
+
+  /**
+   * If current prototype document has not yet finished loading,
+   * appends aDocument to the list of documents to notify (via
+   * PrototypeDocumentContentSink::OnPrototypeLoadDone()) and
+   * sets aLoaded to false. Otherwise sets aLoaded to true.
+   */
+  nsresult AwaitLoadDone(Callback&& aCallback, bool* aResult);
+
+  /**
+   * Notifies each document registered via AwaitLoadDone on this
+   * prototype document that the prototype has finished loading.
+   * The notification is performed by calling
+   * PrototypeDocumentContentSink::OnPrototypeLoadDone on the
+   * registered documents.
+   */
+  nsresult NotifyLoadDone();
+
+  nsNodeInfoManager* GetNodeInfoManager();
+
+  void MarkInCCGeneration(uint32_t aCCGeneration);
+
+  NS_DECL_CYCLE_COLLECTION_CLASS(nsXULPrototypeDocument)
+
+  bool WasL10nCached() { return mWasL10nCached; };
+
+  void SetIsL10nCached(bool aIsCached);
+  void RebuildPrototypeFromElement(nsXULPrototypeElement* aPrototype,
+                                   mozilla::dom::Element* aElement, bool aDeep);
+  void RebuildL10nPrototype(mozilla::dom::Element* aElement, bool aDeep);
+
+ protected:
+  nsCOMPtr<nsIURI> mURI;
+  RefPtr<nsXULPrototypeElement> mRoot;
+  nsTArray<RefPtr<nsXULPrototypePI> > mProcessingInstructions;
+
+  bool mLoaded;
+  nsTArray<Callback> mPrototypeWaiters;
+
+  RefPtr<nsNodeInfoManager> mNodeInfoManager;
+
+  uint32_t mCCGeneration;
+
+  nsXULPrototypeDocument();
+  virtual ~nsXULPrototypeDocument();
+  nsresult Init();
+
+  friend NS_IMETHODIMP NS_NewXULPrototypeDocument(
+      nsXULPrototypeDocument** aResult);
+
+  static uint32_t gRefCnt;
+  bool mWasL10nCached;
+};
+
+#endif  // nsXULPrototypeDocument_h__
diff --git a/dom/xul/nsXULSortService.cpp b/dom/xul/nsXULSortService.cpp
new file mode 100644
index 0000000000..a968ed99d6
--- /dev/null
+++ b/dom/xul/nsXULSortService.cpp
@@ -0,0 +1,387 @@
+/* -*- 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/. */
+
+/*
+  This file provides the implementation for the sort service manager.
+ */
+
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsIContent.h"
+#include "nsGkAtoms.h"
+#include "nsNameSpaceManager.h"
+#include "nsXULContentUtils.h"
+#include "nsString.h"
+#include "nsWhitespaceTokenizer.h"
+#include "nsXULSortService.h"
+#include "nsXULElement.h"
+#include "nsTArray.h"
+#include "nsUnicharUtils.h"
+
+#include "mozilla/dom/Element.h"
+#include "mozilla/intl/Collator.h"
+
+using mozilla::dom::Element;
+const unsigned long SORT_COMPARECASE = 0x0001;
+const unsigned long SORT_INTEGER = 0x0100;
+
+enum nsSortState_direction {
+  nsSortState_descending,
+  nsSortState_ascending,
+  nsSortState_natural
+};
+
+// the sort state holds info about the current sort
+struct MOZ_STACK_CLASS nsSortState final {
+  bool initialized;
+  MOZ_INIT_OUTSIDE_CTOR bool invertSort;
+
+  uint32_t sortHints;
+
+  MOZ_INIT_OUTSIDE_CTOR nsSortState_direction direction;
+  nsAutoString sort;
+  nsTArray<RefPtr<nsAtom>> sortKeys;
+
+  nsCOMPtr<nsIContent> lastContainer;
+  MOZ_INIT_OUTSIDE_CTOR bool lastWasFirst, lastWasLast;
+
+  nsSortState() : initialized(false), sortHints(0) {}
+};
+
+// Information about a particular item to be sorted.
+// Its lifecycle is bound to the 'SortContainer' function scope,
+// so we can use raw pointers to avoid refcount noise during sorting.
+struct contentSortInfo {
+  nsIContent* content;
+  nsIContent* parent;
+};
+
+/**
+ * Set sortActive and sortDirection attributes on a tree column when a sort
+ * is done. The column to change is the one with a sort attribute that
+ * matches the sort key. The sort attributes are removed from the other
+ * columns.
+ */
+static void SetSortColumnHints(nsIContent* content,
+                               const nsAString& sortResource,
+                               const nsAString& sortDirection) {
+  // set sort info on current column. This ensures that the
+  // column header sort indicator is updated properly.
+  for (nsIContent* child = content->GetFirstChild(); child;
+       child = child->GetNextSibling()) {
+    if (child->IsXULElement(nsGkAtoms::treecols)) {
+      SetSortColumnHints(child, sortResource, sortDirection);
+    } else if (child->IsXULElement(nsGkAtoms::treecol)) {
+      nsAutoString value;
+      child->AsElement()->GetAttr(nsGkAtoms::sort, value);
+      if (value == sortResource) {
+        child->AsElement()->SetAttr(kNameSpaceID_None, nsGkAtoms::sortActive,
+                                    u"true"_ns, true);
+
+        child->AsElement()->SetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection,
+                                    sortDirection, true);
+        // Note: don't break out of loop; want to set/unset
+        // attribs on ALL sort columns
+      } else if (!value.IsEmpty()) {
+        child->AsElement()->UnsetAttr(kNameSpaceID_None, nsGkAtoms::sortActive,
+                                      true);
+        child->AsElement()->UnsetAttr(kNameSpaceID_None,
+                                      nsGkAtoms::sortDirection, true);
+      }
+    }
+  }
+}
+
+/**
+ * Set sort and sortDirection attributes when a sort is done.
+ */
+static void SetSortHints(Element* aElement, nsSortState* aSortState) {
+  // set sort and sortDirection attributes when is sort is done
+  aElement->SetAttr(kNameSpaceID_None, nsGkAtoms::sort, aSortState->sort, true);
+
+  nsAutoString direction;
+  if (aSortState->direction == nsSortState_descending)
+    direction.AssignLiteral("descending");
+  else if (aSortState->direction == nsSortState_ascending)
+    direction.AssignLiteral("ascending");
+  aElement->SetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection, direction,
+                    true);
+
+  // for trees, also set the sort info on the currently sorted column
+  if (aElement->IsXULElement(nsGkAtoms::tree)) {
+    if (aSortState->sortKeys.Length() >= 1) {
+      nsAutoString sortkey;
+      aSortState->sortKeys[0]->ToString(sortkey);
+      SetSortColumnHints(aElement, sortkey, direction);
+    }
+  }
+}
+
+/**
+ * Determine the list of items which need to be sorted. This is determined
+ * in the following way:
+ *   - for elements that have a content builder, get its list of generated
+ *     results
+ *   - otherwise, for trees, get the child treeitems
+ *   - otherwise, get the direct children
+ */
+static nsresult GetItemsToSort(nsIContent* aContainer,
+                               nsTArray<contentSortInfo>& aSortItems) {
+  // Get the children. For trees, get the treechildren element and
+  // use that as the parent
+  RefPtr<Element> treechildren;
+  if (aContainer->IsXULElement(nsGkAtoms::tree)) {
+    nsXULContentUtils::FindChildByTag(aContainer, kNameSpaceID_XUL,
+                                      nsGkAtoms::treechildren,
+                                      getter_AddRefs(treechildren));
+    if (!treechildren) return NS_OK;
+
+    aContainer = treechildren;
+  }
+
+  for (nsIContent* child = aContainer->GetFirstChild(); child;
+       child = child->GetNextSibling()) {
+    contentSortInfo* cinfo = aSortItems.AppendElement();
+    if (!cinfo) return NS_ERROR_OUT_OF_MEMORY;
+
+    cinfo->content = child;
+  }
+
+  return NS_OK;
+}
+
+/**
+ * Compares aLeft and aRight and returns < 0, 0, or > 0. The sort
+ * hints are checked for case matching and integer sorting.
+ */
+static int32_t CompareValues(const nsAString& aLeft, const nsAString& aRight,
+                             uint32_t aSortHints) {
+  if (aSortHints & SORT_INTEGER) {
+    nsresult err;
+    int32_t leftint = PromiseFlatString(aLeft).ToInteger(&err);
+    if (NS_SUCCEEDED(err)) {
+      int32_t rightint = PromiseFlatString(aRight).ToInteger(&err);
+      if (NS_SUCCEEDED(err)) {
+        return leftint - rightint;
+      }
+    }
+    // if they aren't integers, just fall through and compare strings
+  }
+
+  if (aSortHints & SORT_COMPARECASE) {
+    return ::Compare(aLeft, aRight);
+  }
+
+  using mozilla::intl::Collator;
+  const Collator* collator = nsXULContentUtils::GetCollator();
+  if (collator) {
+    return collator->CompareStrings(aLeft, aRight);
+  }
+
+  return ::Compare(aLeft, aRight, nsCaseInsensitiveStringComparator);
+}
+
+static int testSortCallback(const contentSortInfo& left,
+                            const contentSortInfo& right,
+                            nsSortState& sortState) {
+  int32_t sortOrder = 0;
+
+  size_t length = sortState.sortKeys.Length();
+  for (size_t t = 0; t < length; t++) {
+    // compare attributes. Ignore namespaces for now.
+    nsAutoString leftstr, rightstr;
+    if (left.content->IsElement()) {
+      left.content->AsElement()->GetAttr(sortState.sortKeys[t], leftstr);
+    }
+    if (right.content->IsElement()) {
+      right.content->AsElement()->GetAttr(sortState.sortKeys[t], rightstr);
+    }
+
+    sortOrder = CompareValues(leftstr, rightstr, sortState.sortHints);
+  }
+
+  if (sortState.direction == nsSortState_descending) sortOrder = -sortOrder;
+
+  return sortOrder;
+}
+
+/**
+ * Given a list of sortable items, reverse the list. This is done
+ * when simply changing the sort direction for the same key.
+ */
+static nsresult InvertSortInfo(nsTArray<contentSortInfo>& aData, int32_t aStart,
+                               int32_t aNumItems) {
+  if (aNumItems > 1) {
+    // reverse the items in the array starting from aStart
+    int32_t upPoint = (aNumItems + 1) / 2 + aStart;
+    int32_t downPoint = (aNumItems - 2) / 2 + aStart;
+    int32_t half = aNumItems / 2;
+    while (half-- > 0) {
+      std::swap(aData[downPoint--], aData[upPoint++]);
+    }
+  }
+  return NS_OK;
+}
+
+/**
+ * Sort a container using the supplied sort state details.
+ */
+static nsresult SortContainer(nsIContent* aContainer, nsSortState& aSortState) {
+  nsTArray<contentSortInfo> items;
+  nsresult rv = GetItemsToSort(aContainer, items);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  uint32_t numResults = items.Length();
+  if (!numResults) return NS_OK;
+
+  uint32_t i;
+
+  // if the items are just being inverted, that is, just switching between
+  // ascending and descending, just reverse the list.
+  if (aSortState.invertSort) {
+    InvertSortInfo(items, 0, numResults);
+  } else {
+    items.Sort([&aSortState](auto left, auto right) {
+      return testSortCallback(left, right, aSortState);
+    });
+  }
+
+  // first remove the items from the old positions
+  for (i = 0; i < numResults; i++) {
+    nsIContent* child = items[i].content;
+    nsIContent* parent = child->GetParent();
+
+    if (parent) {
+      // remember the parent so that it can be reinserted back
+      // into the same parent. This is necessary as multiple rules
+      // may generate results which get placed in different locations.
+      items[i].parent = parent;
+      parent->RemoveChildNode(child, true);
+    }
+  }
+
+  // now add the items back in sorted order
+  for (i = 0; i < numResults; i++) {
+    nsIContent* child = items[i].content;
+    nsIContent* parent = items[i].parent;
+    if (parent) {
+      parent->AppendChildTo(child, true, mozilla::IgnoreErrors());
+
+      // if it's a container in a tree or menu, find its children,
+      // and sort those also
+      if (!child->IsElement() || !child->AsElement()->AttrValueIs(
+                                     kNameSpaceID_None, nsGkAtoms::container,
+                                     nsGkAtoms::_true, eCaseMatters))
+        continue;
+
+      for (nsIContent* grandchild = child->GetFirstChild(); grandchild;
+           grandchild = grandchild->GetNextSibling()) {
+        mozilla::dom::NodeInfo* ni = grandchild->NodeInfo();
+        nsAtom* localName = ni->NameAtom();
+        if (ni->NamespaceID() == kNameSpaceID_XUL &&
+            (localName == nsGkAtoms::treechildren ||
+             localName == nsGkAtoms::menupopup)) {
+          SortContainer(grandchild, aSortState);
+        }
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+/**
+ * Initialize sort information from attributes specified on the container,
+ * the sort key and sort direction.
+ *
+ * @param aRootElement the element that contains sort attributes
+ * @param aContainer the container to sort, usually equal to aRootElement
+ * @param aSortKey space separated list of sort keys
+ * @param aSortDirection direction to sort in
+ * @param aSortState structure filled in with sort data
+ */
+static nsresult InitializeSortState(Element* aRootElement, Element* aContainer,
+                                    const nsAString& aSortKey,
+                                    const nsAString& aSortHints,
+                                    nsSortState* aSortState) {
+  // used as an optimization for the content builder
+  if (aContainer != aSortState->lastContainer.get()) {
+    aSortState->lastContainer = aContainer;
+    aSortState->lastWasFirst = false;
+    aSortState->lastWasLast = false;
+  }
+
+  // The sort attribute is of the form: sort="key1 key2 ..."
+  nsAutoString sort(aSortKey);
+  aSortState->sortKeys.Clear();
+  nsWhitespaceTokenizer tokenizer(sort);
+  while (tokenizer.hasMoreTokens()) {
+    RefPtr<nsAtom> keyatom = NS_Atomize(tokenizer.nextToken());
+    NS_ENSURE_TRUE(keyatom, NS_ERROR_OUT_OF_MEMORY);
+    aSortState->sortKeys.AppendElement(keyatom);
+  }
+
+  aSortState->sort.Assign(sort);
+  aSortState->direction = nsSortState_natural;
+
+  bool noNaturalState = false;
+  nsWhitespaceTokenizer hintsTokenizer(aSortHints);
+  while (hintsTokenizer.hasMoreTokens()) {
+    const nsDependentSubstring& token(hintsTokenizer.nextToken());
+    if (token.EqualsLiteral("comparecase"))
+      aSortState->sortHints |= SORT_COMPARECASE;
+    else if (token.EqualsLiteral("integer"))
+      aSortState->sortHints |= SORT_INTEGER;
+    else if (token.EqualsLiteral("descending"))
+      aSortState->direction = nsSortState_descending;
+    else if (token.EqualsLiteral("ascending"))
+      aSortState->direction = nsSortState_ascending;
+    else if (token.EqualsLiteral("twostate"))
+      noNaturalState = true;
+  }
+
+  // if the twostate flag was set, the natural order is skipped and only
+  // ascending and descending are allowed
+  if (aSortState->direction == nsSortState_natural && noNaturalState) {
+    aSortState->direction = nsSortState_ascending;
+  }
+
+  // set up sort order info
+  aSortState->invertSort = false;
+
+  nsAutoString existingsort;
+  aRootElement->GetAttr(nsGkAtoms::sort, existingsort);
+  nsAutoString existingsortDirection;
+  aRootElement->GetAttr(nsGkAtoms::sortDirection, existingsortDirection);
+
+  // if just switching direction, set the invertSort flag
+  if (sort.Equals(existingsort)) {
+    if (aSortState->direction == nsSortState_descending) {
+      if (existingsortDirection.EqualsLiteral("ascending"))
+        aSortState->invertSort = true;
+    } else if (aSortState->direction == nsSortState_ascending &&
+               existingsortDirection.EqualsLiteral("descending")) {
+      aSortState->invertSort = true;
+    }
+  }
+
+  aSortState->initialized = true;
+
+  return NS_OK;
+}
+
+nsresult mozilla::XULWidgetSort(Element* aNode, const nsAString& aSortKey,
+                                const nsAString& aSortHints) {
+  nsSortState sortState;
+  nsresult rv =
+      InitializeSortState(aNode, aNode, aSortKey, aSortHints, &sortState);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // store sort info in attributes on content
+  SetSortHints(aNode, &sortState);
+  rv = SortContainer(aNode, sortState);
+
+  return rv;
+}
diff --git a/dom/xul/nsXULSortService.h b/dom/xul/nsXULSortService.h
new file mode 100644
index 0000000000..c087cb572a
--- /dev/null
+++ b/dom/xul/nsXULSortService.h
@@ -0,0 +1,42 @@
+/* -*- 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/. */
+
+/*
+  This sort service is used to sort content by attribute.
+ */
+
+#ifndef nsXULSortService_h
+#define nsXULSortService_h
+
+#include "nsAString.h"
+#include "nsError.h"
+
+namespace mozilla {
+
+namespace dom {
+class Element;
+}  // namespace dom
+
+/**
+ * Sort the contents of the widget containing <code>aNode</code>
+ * using <code>aSortKey</code> as the comparison key, and
+ * <code>aSortDirection</code> as the direction.
+ *
+ * @param aNode A node in the XUL widget whose children are to be sorted.
+ * @param aSortKey The value to be used as the comparison key.
+ * @param aSortHints One or more hints as to how to sort:
+ *
+ *   ascending: to sort the contents in ascending order
+ *   descending: to sort the contents in descending order
+ *   comparecase: perform case sensitive comparisons
+ *   integer: treat values as integers, non-integers are compared as strings
+ *   twostate: don't allow the natural (unordered state)
+ */
+nsresult XULWidgetSort(dom::Element* aNode, const nsAString& aSortKey,
+                       const nsAString& aSortHints);
+
+}  // namespace mozilla
+
+#endif  // nsXULSortService_h
diff --git a/dom/xul/test/398289-resource.xhtml b/dom/xul/test/398289-resource.xhtml
new file mode 100644
index 0000000000..6702027ef1
--- /dev/null
+++ b/dom/xul/test/398289-resource.xhtml
@@ -0,0 +1,49 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<dialog buttonlabelaccept="OK"
+        buttonlabelcancel="Cancel">
+  <style xmlns="http://www.w3.org/1999/xhtml">
+    .tab-middle { outline: none !important }
+  </style>
+
+  <script>
+    document.addEventListener("dialogaccept", function() { alert('OK') });
+    document.addEventListener("dialogcancel", function() { alert('Cancel') });
+  </script>
+
+  <tabbox id="test" flex="1" persist="selectedIndex">
+
+    <tabs>
+      <tab label="One"/>
+      <tab label="Two"/>
+    </tabs>
+
+    <tabpanels flex="1">
+
+      <vbox flex="1">
+        <description>Text for tab ONE</description>
+        <description class="text-link"
+                     onclick="window.open('https://bugzilla.mozilla.org/show_bug.cgi?id=398289');">(test case for bug 398289)</description>
+        <tree>
+          <treecols>
+            <treecol label="Header" flex="1"/>
+          </treecols>
+        </tree>
+      </vbox>
+
+      <vbox flex="1">
+        <description>Text for tab TWO</description>
+        <description>(When the document is reloaded, this content gets replaced by the one from the first tab.)</description>
+      </vbox>
+
+    </tabpanels>
+
+  </tabbox>
+
+  <box height="1000"/> <!-- Push dialog buttons out of sight so that the animated default button isn't part of the snapshot -->
+
+</dialog>
+</window>
diff --git a/dom/xul/test/chrome.toml b/dom/xul/test/chrome.toml
new file mode 100644
index 0000000000..3e239fe1d9
--- /dev/null
+++ b/dom/xul/test/chrome.toml
@@ -0,0 +1,42 @@
+[DEFAULT]
+support-files = [
+  "398289-resource.xhtml",
+  "window_bug583948.xhtml",
+]
+
+["test_accesskey.xhtml"]
+
+["test_bug199692.xhtml"]
+
+["test_bug311681.xhtml"]
+
+["test_bug391002.xhtml"]
+
+["test_bug398289.html"]
+
+["test_bug403868.xhtml"]
+
+["test_bug418216.xhtml"]
+
+["test_bug445177.xhtml"]
+
+["test_bug468176.xhtml"]
+
+["test_bug583948.xhtml"]
+
+["test_bug749367.xhtml"]
+
+["test_bug775972.xhtml"]
+
+["test_bug1070049_throw_from_script.xhtml"]
+
+["test_bug1290965.xhtml"]
+
+["test_bug1686822.xhtml"]
+support-files = ["window_bug1686822.xhtml"]
+
+["test_html_template.xhtml"]
+
+["test_import_xul_to_content.xhtml"]
+
+["test_xul_tabindex_focus.xhtml"]
diff --git a/dom/xul/test/file_bug236853.rdf b/dom/xul/test/file_bug236853.rdf
new file mode 100644
index 0000000000..8d17bf6912
--- /dev/null
+++ b/dom/xul/test/file_bug236853.rdf
@@ -0,0 +1,14 @@
+<?xml version="1.0"?>
+<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+	xmlns:ex="http://www.ex.org/ex-rdf#">
+
+	<rdf:Description about="urn:root">
+		<ex:nodes>
+			<rdf:Bag about="http://www.ex.org/nodes">
+				<rdf:li>
+					<rdf:Description about="http://www.ex.org/nodes/A"/>
+				</rdf:li>
+			</rdf:Bag>
+		</ex:nodes>
+	</rdf:Description>		
+</rdf:RDF>
diff --git a/dom/xul/test/mochitest.toml b/dom/xul/test/mochitest.toml
new file mode 100644
index 0000000000..70c4690b01
--- /dev/null
+++ b/dom/xul/test/mochitest.toml
@@ -0,0 +1,3 @@
+[DEFAULT]
+
+["test_disable_scroll_frame_plain.html"]
diff --git a/dom/xul/test/test_accesskey.xhtml b/dom/xul/test/test_accesskey.xhtml
new file mode 100644
index 0000000000..c71fcf78a0
--- /dev/null
+++ b/dom/xul/test/test_accesskey.xhtml
@@ -0,0 +1,57 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1699284
+-->
+<window title="Mozilla Bug 1699284"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+<script src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"></script>
+<body>
+<label id="label1" accesskey="a">Label 1</label><button id="button1" accesskey="a">Button 1</button>
+<label id="label2" accesskey="a" tabindex="0">Label 2</label><button id="button2">Button 2</button>
+<label id="label3">Label 3</label><button id="button3" accesskey="a">Button 3</button>
+<label id="label4" accesskey="a" control="button4">Label 4</label><button id="button4" disabled="true">Button 4</button>
+<label id="label5" accesskey="a" control="button5">Label 5</label><button id="button5">Button 5</button>
+<!-- Tests code -->
+<script type="application/javascript">
+<![CDATA[
+
+/** Test for Bug 1699284 **/
+
+function PerformAccessKey(aKey) {
+  synthesizeKey(aKey, navigator.platform.includes("Mac") ? { altKey: true, ctrlKey: true }
+                                                         : { altKey: true, shiftKey: true });
+};
+
+add_task(async function test_accesskey() {
+  let [label1, label2, label3, label4, label5] = document.querySelectorAll("label");
+  let [button1, button2, button3, button4, button5] = document.querySelectorAll("button");
+
+  [
+    label1, label2, label3, label4, label5, button1, button2, button3, button4, button5
+  ].forEach(function(ele) {
+    ele.addEventListener("click", function(e) {
+      ok(false, `${e.target.id} should not receive click event`);
+    });
+  });
+
+  PerformAccessKey("a");
+  is(document.activeElement.id, button1.id, `focus should move to ${button1.id}`);
+
+  PerformAccessKey("a");
+  is(document.activeElement.id, button3.id, `focus should move to ${button3.id}`);
+
+  PerformAccessKey("a");
+  is(document.activeElement.id, button5.id, `focus should move to ${button5.id}`);
+
+  // Cycle back to first element
+  PerformAccessKey("a");
+  is(document.activeElement.id, button1.id, `focus should move to ${button1.id}`);
+});
+
+]]>
+</script>
+</body>
+</window>
diff --git a/dom/xul/test/test_bug1070049_throw_from_script.xhtml b/dom/xul/test/test_bug1070049_throw_from_script.xhtml
new file mode 100644
index 0000000000..d5fb83e6ff
--- /dev/null
+++ b/dom/xul/test/test_bug1070049_throw_from_script.xhtml
@@ -0,0 +1,40 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1070049
+-->
+<window title="Mozilla Bug 1070049"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+
+  <!-- test code goes here -->
+  <script type="application/javascript">
+  <![CDATA[
+
+  /** Test for Bug 1070049 **/
+  SimpleTest.waitForExplicitFinish();
+  addLoadEvent(function() {
+    // Prevent the test from failing when the exception hits onerror.
+    SimpleTest.expectUncaughtException();
+
+    // Tell the test to expect exactly one console error with the given parameters,
+    // with SimpleTest.finish as a continuation function.
+    SimpleTest.monitorConsole(SimpleTest.finish, [{errorMessage: new RegExp('flimfniffle')}]);
+
+    // Schedule the console accounting (and continuation) to run next, right
+    // after we throw (below).
+    SimpleTest.executeSoon(SimpleTest.endMonitorConsole);
+
+    // Throw.
+    throw new Error("flimfniffle");
+  });
+  ]]>
+  </script>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1070049"
+     target="_blank">Mozilla Bug 1070049</a>
+  </body>
+</window>
diff --git a/dom/xul/test/test_bug1290965.xhtml b/dom/xul/test/test_bug1290965.xhtml
new file mode 100644
index 0000000000..dbef1df844
--- /dev/null
+++ b/dom/xul/test/test_bug1290965.xhtml
@@ -0,0 +1,38 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:html="http://www.w3.org/1999/xhtml">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+  <toolbarbutton oncommand="++countera;" id="a">A</toolbarbutton>
+  <toolbarbutton oncommand="++counterb;" id="b">B</toolbarbutton>
+  <script>
+  //<![CDATA[
+    let aEl = document.getElementById('a');
+    let bEl = document.getElementById('b');
+    let countera = 0;
+    let counterb = 0;
+
+    aEl.addEventListener('click', function (aEvent) {
+      aEvent.preventDefault();
+      let cmdEvent = document.createEvent("xulcommandevent");
+      cmdEvent.initCommandEvent("command", true, true, window, 0,
+                                aEvent.ctrlKey, aEvent.altKey, aEvent.shiftKey,
+                                aEvent.metaKey, 0, null, aEvent.inputSource);
+      aEvent.currentTarget.dispatchEvent(cmdEvent);
+    });
+
+    bEl.addEventListener('click', function (aEvent) {
+      let cmdEvent = document.createEvent("xulcommandevent");
+      cmdEvent.initCommandEvent("command", true, true, window, 0,
+                                aEvent.ctrlKey, aEvent.altKey, aEvent.shiftKey,
+                                aEvent.metaKey, 0, null, aEvent.inputSource);
+      aEvent.currentTarget.dispatchEvent(cmdEvent);
+    });
+
+    bEl.click();
+    aEl.click();
+
+    is(countera, 1, "Counter should be one as event fires once");
+    is(counterb, 2, "Counter should be two as event fires twice");
+  //]]>
+  </script>
+</window>
diff --git a/dom/xul/test/test_bug1686822.xhtml b/dom/xul/test/test_bug1686822.xhtml
new file mode 100644
index 0000000000..cdad585d00
--- /dev/null
+++ b/dom/xul/test/test_bug1686822.xhtml
@@ -0,0 +1,35 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:html="http://www.w3.org/1999/xhtml">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+  <html:script><![CDATA[
+    SimpleTest.waitForExplicitFinish();
+    let chromeWindow = browsingContext.topChromeWindow;
+
+    let resolve;
+    let i = 0;
+    chromeWindow.moduleScriptRan = function() {
+      ok(true, "Module script executed: " + ++i)
+      resolve();
+      resolve = null;
+    }
+
+    function testOnce() {
+      let currentWin;
+      return new Promise(r => {
+        currentWin = chromeWindow.open("window_bug1686822.xhtml", "", "chrome");
+        resolve = r;
+      }).then(function() {
+        currentWin.close();
+      });
+    }
+
+    (async function() {
+      // The first and second loads are different so make sure we test both code paths.
+      await testOnce();
+      await testOnce();
+      delete chromeWindow.moduleScriptRan;
+      SimpleTest.finish();
+    }());
+  ]]></html:script>
+</window>
diff --git a/dom/xul/test/test_bug199692.xhtml b/dom/xul/test/test_bug199692.xhtml
new file mode 100644
index 0000000000..6d8b9efe4e
--- /dev/null
+++ b/dom/xul/test/test_bug199692.xhtml
@@ -0,0 +1,102 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=199692
+-->
+<window title="Test for Bug 199692"
+  id="test_bug199692.xhtml"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+  xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript">
+  <![CDATA[
+  customElements.define("test-xul-element", class CustomElement extends XULElement {
+    constructor() {
+      super();
+      const template = document.getElementById("template");
+      this.attachShadow({mode: "open"})
+        .appendChild(template.content.cloneNode(true));
+    }
+  });
+  ]]>
+  </script>
+<body id="body" xmlns="http://www.w3.org/1999/xhtml">
+<template id="template">
+  <xul:label id="anon-label" value="ANON"/>
+</template>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=199692">Mozilla Bug 199692</a>
+
+<vbox id="content" style="position: relative;"
+xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <xul:label id="non-anon-label" value="a textbox!:" control="textbox"/>
+  <html:textarea id="textbox" rows="4"/>
+  <xul:radiogroup style="outline: 2px solid orange;">
+    <xul:radio id="unselected-radio" label="Orange" style="outline: 2px solid red;"/>
+    <xul:radio id="selected-radio" label="Violet" selected="true"/>
+    <xul:radio id="disabled-radio" label="Yellow" disabled="true"/>
+  </xul:radiogroup>
+  <test-xul-element id="bound" style="border: 2px solid green;"></test-xul-element>
+</vbox>
+<pre id="test">
+  <script class="testbody" type="text/javascript">
+<![CDATA[
+  SimpleTest.waitForExplicitFinish();
+
+  // Before onload, XUL docs have no root frame.
+  is(document.elementFromPoint(10,10), null,
+     "Calls to elementFromPoint before onload should return null");
+
+  var d = 10;
+  function middle(e) {
+    return { "x": e.getBoundingClientRect().x + e.getBoundingClientRect().width/2,
+             "y": e.getBoundingClientRect().y + e.getBoundingClientRect().height/2 };
+  }
+  function lower_right(e) {
+    return { "x": e.getBoundingClientRect().x + e.getBoundingClientRect().width - d,
+             "y": e.getBoundingClientRect().y + e.getBoundingClientRect().height - d };
+  }
+  function upper_left(e) {
+    return { "x": e.getBoundingClientRect().x + d,
+             "y": e.getBoundingClientRect().y + d };
+  }
+  function scrollbar_button(e) { // a bit down from upper right
+    return { "x": e.getBoundingClientRect().x + e.getBoundingClientRect().width - d,
+             "y": e.getBoundingClientRect().y + d + 15 };
+  }
+
+  function test(ptFunc, id, message) {
+    var pt = ptFunc($(id));
+    var e = document.elementFromPoint(pt.x, pt.y);
+    ok(e != null, message + " (returned null)");
+    is(e.id, id, message);
+  }
+
+  function do_test() {
+    // Avoid hardcoding x,y pixel values, to better deal with differing default
+    // font sizes or other layout defaults.
+
+    test(middle, 'textbox', "Point within textbox should return textbox element");
+    test(lower_right, 'textbox', "Point on textbox's scrollbar should return textbox element");
+    test(scrollbar_button, 'textbox', "Point on textbox's scrollbar button should return textbox element");
+    test(middle, 'non-anon-label', "Point on label should return label");
+    test(upper_left, 'bound', "Point on custom element content should return custom element");
+
+    SimpleTest.finish();
+  }
+  $("textbox").setAttribute("value",
+      "lorem ipsum dolor sit amet " +
+      "lorem ipsum dolor sit amet " +
+      "lorem ipsum dolor sit amet " +
+      "lorem ipsum dolor sit amet " +
+      "lorem ipsum dolor sit amet " +
+      "lorem ipsum dolor sit amet " +
+      "lorem ipsum dolor sit amet "); // force scrollbars to appear
+  addLoadEvent(do_test);
+]]>
+  </script>
+</pre>
+</body>
+</window>
diff --git a/dom/xul/test/test_bug311681.xhtml b/dom/xul/test/test_bug311681.xhtml
new file mode 100644
index 0000000000..8caef2ca5b
--- /dev/null
+++ b/dom/xul/test/test_bug311681.xhtml
@@ -0,0 +1,107 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=311681
+-->
+<window title="Mozilla Bug 311681"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>      
+
+<body  xmlns="http://www.w3.org/1999/xhtml">
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=311681">Mozilla Bug 311681</a>
+<script class="testbody" type="text/javascript">
+<![CDATA[
+  // Setup script
+  SimpleTest.waitForExplicitFinish();
+
+  // Make sure to trigger the hashtable case by asking for enough elements
+  // by ID.
+  for (var i = 0; i < 256; ++i) {
+    var x = document.getElementById(i);
+  }
+
+  // save off the document.getElementById function, since getting it as a
+  // property off the document it causes a content flush.      
+  var fun = document.getElementById;
+
+  // Slot for our initial element with id "content"
+  var testNode;
+
+  function getCont() {
+    return fun.call(document, "content");
+  }
+  
+  function testClone() {
+    // Test to make sure that if we have multiple nodes with the same ID in
+    // a document we don't forget about one of them when the other is
+    // removed.
+    var newParent = $("display");
+    var node = testNode.cloneNode(true);
+    isnot(node, testNode, "Clone should be a different node");
+
+    newParent.appendChild(node);
+
+    // Check what getElementById returns, no flushing
+    is(getCont(), node, "Should be getting new node pre-flush 1")
+    
+    // Trigger a layout flush, just in case.
+    let itemHeight = newParent.offsetHeight/10;
+
+    // Check what getElementById returns now.
+    is(getCont(), node, "Should be getting new node post-flush 1")
+    
+    clear(newParent);
+    
+    // Check what getElementById returns, no flushing
+    is(getCont(), testNode, "Should be getting orig node pre-flush 2");
+    
+    // Trigger a layout flush, just in case.
+    itemHeight = newParent.offsetHeight/10;
+
+    // Check what getElementById returns now.
+    is(getCont(), testNode, "Should be getting orig node post-flush 2");
+
+    node = testNode.cloneNode(true);
+    newParent.appendChild(node);
+    testNode.remove();
+
+    // Check what getElementById returns, no flushing
+    is(getCont(), node, "Should be getting clone pre-flush");
+    
+    // Trigger a layout flush, just in case.
+    // eslint-disable-next-line no-unused-vars
+    itemHeight = newParent.offsetHeight/10;
+
+    // Check what getElementById returns now.
+    is(getCont(), node, "Should be getting clone post-flush");
+
+  }
+
+  function clear(node) {
+    while (node.hasChildNodes()) {
+      node.firstChild.remove();
+    }
+  }
+
+  addLoadEvent(testClone);
+  addLoadEvent(SimpleTest.finish);
+]]>
+</script>
+<p id="display"></p>
+<div id="content" style="display: none">
+  <script class="testbody" type="text/javascript">
+  <![CDATA[
+    testNode = fun.call(document, "content");
+    // Needs incremental XML parser
+    isnot(testNode, null, "Should have node here");
+  ]]>
+  </script>  
+</div>
+<pre id="test">
+</pre>
+</body>
+
+</window>
diff --git a/dom/xul/test/test_bug391002.xhtml b/dom/xul/test/test_bug391002.xhtml
new file mode 100644
index 0000000000..da8c8b4d35
--- /dev/null
+++ b/dom/xul/test/test_bug391002.xhtml
@@ -0,0 +1,41 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=391002
+-->
+<window title="Mozilla Bug 391002"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=391002"
+     target="_blank">Mozilla Bug 391002</a>
+  </body>
+
+  <button id="btn1" command="cmd1"/>
+
+  <button id="btn2">
+    <observes id="observes" element="cmd1" attribute="label"/>
+  </button>
+
+  <commandset>
+    <command id="cmd1" label="cmd1"/>
+    <command id="cmd2" label="cmd2"/>
+  </commandset>
+
+  <!-- test code goes here -->
+  <script type="application/javascript"><![CDATA[
+
+    /** Test for Bug 391002 **/
+
+    $("btn1").setAttribute("command", "cmd2");
+    is($("btn1").getAttribute("label"), $("cmd2").getAttribute("label"))
+
+    $("observes").setAttribute("element", "cmd2");
+    is($("btn2").getAttribute("label"), $("cmd2").getAttribute("label"))
+
+  ]]></script>
+</window>
+
diff --git a/dom/xul/test/test_bug398289.html b/dom/xul/test/test_bug398289.html
new file mode 100644
index 0000000000..c4873fe2b2
--- /dev/null
+++ b/dom/xul/test/test_bug398289.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<html style="height: 100%">
+<head>
+  <title>Test for bug 398289</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <script src="chrome://mochikit/content/tests/SimpleTest/WindowSnapshot.js"></script>
+  <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css" />
+</head>
+<body style="height: 100%; margin: 0; overflow: hidden;" onload="setTimeout(onBodyLoad, 0);">
+  <iframe id="test" style="border: 0; width: 100%; height: 100%" scrolling="no" src="398289-resource.xhtml"></iframe>
+
+  <script class="testbody">
+    var snap1, snap2;
+    SimpleTest.waitForExplicitFinish();
+
+    async function onBodyLoad() {
+      window.frames[0].document.getElementById("test").selectedIndex = 0;
+      window.frames[0].document.getElementById("test").selectedIndex = 1;
+
+      frames[0].scrollTo(0, 0);
+      scrollTo(0, 0);
+      snap1 = await snapshotWindow(window);
+
+      document.getElementById("test").onload = onFrameLoad;
+      window.frames[0].location.reload();
+    }
+
+    async function onFrameLoad() {
+      frames[0].scrollTo(0, 0);
+      scrollTo(0, 0);
+      snap2 = await snapshotWindow(window);
+
+      var equal, str1, str2;
+      [equal, str1, str2] = compareSnapshots(snap1, snap2, true);
+
+      ok(equal, "persistent attribute in tab box broken, expected: "+str1+" got: "+str2);
+      SimpleTest.finish();
+    }
+  </script>
+</body>
+</html>
diff --git a/dom/xul/test/test_bug403868.xhtml b/dom/xul/test/test_bug403868.xhtml
new file mode 100644
index 0000000000..c2427bb644
--- /dev/null
+++ b/dom/xul/test/test_bug403868.xhtml
@@ -0,0 +1,83 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=403868
+-->
+<window title="Mozilla Bug 403868"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=403868"
+     target="_blank">Mozilla Bug 403868</a>
+    <div id="content" style="display: none"/>
+  </body>
+
+  <!-- test code goes here -->
+  <script type="application/javascript"><![CDATA[
+
+    /** Test for Bug 403868 **/
+function createSpan(id, insertionPoint) {
+  var s = document.createElementNS("http://www.w3.org/1999/xhtml", "span");
+  s.id = id;
+  $("content").insertBefore(s, insertionPoint);
+  return s;
+}
+
+var s1a = createSpan("test1", null);
+is(document.getElementById("test1"), s1a,
+   "Only one span with id=test1 in the tree; should work!");
+
+var s2a = createSpan("test1", null);
+is(document.getElementById("test1"), s1a,
+   "Appending span with id=test1 doesn't change which one comes first");
+
+var s3a = createSpan("test1", s2a);
+is(document.getElementById("test1"), s1a,
+   "Inserting span with id=test1 not at the beginning; doesn't matter");
+
+var s4a = createSpan("test1", s1a);
+is(document.getElementById("test1"), s4a,
+   "Inserting span with id=test1 at the beginning changes which one is first");
+
+s4a.remove();
+is(document.getElementById("test1"), s1a,
+   "First-created span with id=test1 is first again");
+
+s1a.remove();
+is(document.getElementById("test1"), s3a,
+   "Third-created span with id=test1 is first now");
+
+// Start the id hashtable
+for (var i = 0; i < 256; ++i) {
+  document.getElementById("no-such-id-in-the-document" + i);
+}
+
+var s1b = createSpan("test2", null);
+is(document.getElementById("test2"), s1b,
+   "Only one span with id=test2 in the tree; should work!");
+
+var s2b = createSpan("test2", null);
+is(document.getElementById("test2"), s1b,
+   "Appending span with id=test2 doesn't change which one comes first");
+
+var s3b = createSpan("test2", s2b);
+is(document.getElementById("test2"), s1b,
+   "Inserting span with id=test2 not at the beginning; doesn't matter");
+
+var s4b = createSpan("test2", s1b);
+is(document.getElementById("test2"), s4b,
+   "Inserting span with id=test2 at the beginning changes which one is first");
+
+s4b.remove();
+is(document.getElementById("test2"), s1b,
+   "First-created span with id=test2 is first again");
+
+s1b.remove();
+is(document.getElementById("test2"), s3b,
+   "Third-created span with id=test2 is first now");
+
+  ]]></script>
+</window>
diff --git a/dom/xul/test/test_bug418216.xhtml b/dom/xul/test/test_bug418216.xhtml
new file mode 100644
index 0000000000..56e487faf2
--- /dev/null
+++ b/dom/xul/test/test_bug418216.xhtml
@@ -0,0 +1,46 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=418216
+-->
+<window title="Mozilla Bug 418216"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=418216"
+     target="_blank">Mozilla Bug 418216</a>
+  </body>
+
+  <!-- test code goes here -->
+  <script type="application/javascript"><![CDATA[
+
+    /** Test for Bug 418216 **/
+  SimpleTest.waitForExplicitFinish();
+  window.onload = function onload() {
+    var element = $("item");
+    $("container").removeChild($("item"));
+    $("broadcaster").removeAttribute("disabled");
+    $("container").appendChild(element);
+    is($("item").hasAttribute("disabled"), $("broadcaster").hasAttribute("disabled"));
+    SimpleTest.finish();
+  }
+
+
+
+
+  ]]></script>
+
+<box id="container">
+<box id="item">
+    <observes element="broadcaster" attribute="disabled"/>
+</box>
+</box>
+
+<broadcasterset>
+    <broadcaster id="broadcaster" disabled="true"/>
+</broadcasterset>
+
+</window>
diff --git a/dom/xul/test/test_bug445177.xhtml b/dom/xul/test/test_bug445177.xhtml
new file mode 100644
index 0000000000..d188f19241
--- /dev/null
+++ b/dom/xul/test/test_bug445177.xhtml
@@ -0,0 +1,66 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=445177
+-->
+<window title="Test for Bug 445177"
+  id="test_bug445177.xhtml"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+  xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+
+<body id="body" xmlns="http://www.w3.org/1999/xhtml">
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=445177">Mozilla Bug 445177</a>
+
+
+<xul:hbox id="b1" value="foo"/>
+<xul:hbox id="o1" observes="b1"/>
+
+<pre id="test">
+  <script class="testbody" type="text/javascript">
+<![CDATA[
+  SimpleTest.waitForExplicitFinish();
+  function do_test() {
+    var b1 = document.getElementById("b1");
+    var o1 = document.getElementById("o1");
+
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (1)");
+
+    b1.setAttribute("value", "bar");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (2)");
+
+    b1.removeAttribute("value");
+    is(o1.hasAttribute("value"), b1.hasAttribute("value"), "Wrong value (3)");
+
+    o1.setAttribute("value", "foo");
+    isnot(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (4)");
+
+    b1.setAttribute("value", "foobar");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (5)");
+
+    //After removing listener, changes to broadcaster shouldn't have any effect.
+    o1.remove();
+    b1.setAttribute("value", "foo");
+    isnot(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (6)");
+
+    b1.parentNode.appendChild(o1);
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (7)");
+
+    o1.remove();
+    o1.removeAttribute("observes");
+    o1.removeAttribute("value");
+    b1.parentNode.appendChild(o1);
+    isnot(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (8)");
+
+    SimpleTest.finish();
+  }
+
+  addLoadEvent(do_test);
+]]>
+  </script>
+</pre>
+</body>
+</window>
diff --git a/dom/xul/test/test_bug468176.xhtml b/dom/xul/test/test_bug468176.xhtml
new file mode 100644
index 0000000000..f21c1bcde3
--- /dev/null
+++ b/dom/xul/test/test_bug468176.xhtml
@@ -0,0 +1,84 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=468176
+-->
+<window title="Test for Bug 468176"
+  id="test_bug468176.xhtml"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+  xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+
+<body id="body" xmlns="http://www.w3.org/1999/xhtml">
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=468176">Mozilla Bug 468176</a>
+
+<xul:hbox id="b1" value="foo"/>
+
+<xul:hbox id="o1">
+  <xul:observes id="inner" element="b1" attribute="*"/>
+</xul:hbox>
+
+<pre id="test">
+  <script class="testbody" type="text/javascript">
+<![CDATA[
+  SimpleTest.waitForExplicitFinish();
+
+  var broadcastCount = 0;
+  function b_listener(evt) {
+    ++broadcastCount;
+  }
+
+  function do_test() {
+    var b1 = document.getElementById("b1");
+    var o1 = document.getElementById("o1");
+    var inner = document.getElementById("inner");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (1)");
+
+    inner.addEventListener("broadcast", b_listener, true);
+    b1.setAttribute("value", "bar");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (2)");
+    is(broadcastCount, 1, "Wrong value (3)");
+
+    b1.removeAttribute("value");
+    is(o1.hasAttribute("value"), b1.hasAttribute("value"), "Wrong value (4)");
+    is(broadcastCount, 2, "Wrong value (5)");
+
+    o1.setAttribute("value", "foo");
+    isnot(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (6)");
+    is(broadcastCount, 2, "Wrong value (7)");
+
+    b1.setAttribute("value", "foobar");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (8)");
+    is(broadcastCount, 3, "Wrong value (9)");
+
+    b1.removeAttribute("value");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (10)");
+    is(broadcastCount, 4, "Wrong value (11)");
+
+    b1.removeAttribute("value");
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (12)");
+    is(broadcastCount, 4, "Wrong value (13)");
+
+    o1.setAttribute("value", "bar");
+    b1.setAttribute("value", "bar"); // This should still dispatch 'broadcast'
+    is(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (14)");
+    is(broadcastCount, 5, "Wrong value (15)");
+
+    //After removing listener, changes to broadcaster shouldn't have any effect.
+    o1.remove();
+    b1.setAttribute("value", "foo");
+    isnot(o1.getAttribute("value"), b1.getAttribute("value"), "Wrong value (16)");
+    is(broadcastCount, 5, "Wrong value (17)");
+
+    SimpleTest.finish();
+  }
+
+  addLoadEvent(do_test);
+]]>
+  </script>
+</pre>
+</body>
+</window>
diff --git a/dom/xul/test/test_bug583948.xhtml b/dom/xul/test/test_bug583948.xhtml
new file mode 100644
index 0000000000..2cab13a4ac
--- /dev/null
+++ b/dom/xul/test/test_bug583948.xhtml
@@ -0,0 +1,41 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css"
+                 type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+<script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+
+<body xmlns="http://www.w3.org/1999/xhtml">
+  <div id="content" style="display: none"/>
+</body>
+
+<script>
+SimpleTest.waitForExplicitFinish();
+
+let chromeWindow = window.browsingContext.topChromeWindow;
+
+var attempts = 0;
+
+chromeWindow.update = function () {
+  // without the crash fix, this usually crashes after 2 to 4 reloads
+  if (++attempts == 6) {
+    ok(true, "didn't crash after 6 attempts");
+    otherWindow.close();
+    SimpleTest.waitForFocus(function() {
+      chromeWindow.update = null;
+      SimpleTest.finish();
+    });
+  } else {
+    otherWindow.document.commandDispatcher.updateCommands('');
+    setTimeout(function() {
+      otherWindow.location.reload()
+    }, 0);
+  }
+}
+
+var otherWindow = chromeWindow.open("window_bug583948.xhtml", "_blank", "chrome");
+</script>
+
+</window>
diff --git a/dom/xul/test/test_bug749367.xhtml b/dom/xul/test/test_bug749367.xhtml
new file mode 100644
index 0000000000..3dcea24f1c
--- /dev/null
+++ b/dom/xul/test/test_bug749367.xhtml
@@ -0,0 +1,39 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=749367
+-->
+<window title="Mozilla Bug 749367"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="setTimeout(runTests, 0);">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=486990"
+     target="_blank">Mozilla Bug 486990</a>
+  </body>
+
+  <!-- test code goes here -->
+  <script type="text/template">
+  <![CDATA[
+  SimpleTest.waitForExplicitFinish();
+  function runTests() {
+  ok(false, "Shouldn't execute");
+  SimpleTest.finish();
+  }
+  ]]>
+  </script>
+
+  <script type="text/javascript">
+  <![CDATA[
+  SimpleTest.waitForExplicitFinish();
+  function runTests() {
+  ok(true, "Should execute");
+  SimpleTest.finish();
+  }
+  ]]>
+  </script>
+
+</window>
diff --git a/dom/xul/test/test_bug775972.xhtml b/dom/xul/test/test_bug775972.xhtml
new file mode 100644
index 0000000000..571bf6a6c3
--- /dev/null
+++ b/dom/xul/test/test_bug775972.xhtml
@@ -0,0 +1,35 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=775972
+-->
+<window title="Mozilla Bug 775972"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test()">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=775972"
+     target="_blank">Mozilla Bug 775972</a>
+  </body>
+
+  <hbox id="container"><label value="test" id=""/></hbox>
+  <!-- test code goes here -->
+  <script type="application/javascript">
+  <![CDATA[
+
+  /** Test for Bug 775972 **/
+
+  function test() {
+    var c = document.getElementById("container");
+    var clone = c.cloneNode(true);
+    document.documentElement.appendChild(clone);
+    ok(true, "This shouldn't crash!");
+  }
+
+
+  ]]>
+  </script>
+</window>
diff --git a/dom/xul/test/test_disable_scroll_frame_plain.html b/dom/xul/test/test_disable_scroll_frame_plain.html
new file mode 100644
index 0000000000..8104f55721
--- /dev/null
+++ b/dom/xul/test/test_disable_scroll_frame_plain.html
@@ -0,0 +1,19 @@
+<!DOCTYPE HTML>
+<html scrolling="false">
+<head>
+  <meta charset="utf-8">
+  <title>disable scroll frame exposed</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<p id="display"></p>
+<div id="content" style="display: none"></div>
+<pre id="test"></pre>
+<div style="height: 300vh"></div>
+<script>
+  // Ensure that disabling the scroll frame isn't exposed to content.
+  ok(document.scrollingElement.scrollTopMax > 0, "Scrolling should still work.");
+</script>
+</body>
+</html>
diff --git a/dom/xul/test/test_html_template.xhtml b/dom/xul/test/test_html_template.xhtml
new file mode 100644
index 0000000000..7e6029486e
--- /dev/null
+++ b/dom/xul/test/test_html_template.xhtml
@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"?>
+<window title="HTML template in XUL"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <template id="template">Content<span>Content</span></template>
+<script type="application/javascript"><![CDATA[
+  add_task(async function test_template() {
+    let template = document.getElementById("template");
+    ok("content" in template, "Template has shadow root.");
+    is(template.childNodes.length, 0, "Template should have no children.");
+    is(template.content.childNodes.length, 2, "Template content should have two children.");
+  });
+]]></script>
+  </body>
+</window>
diff --git a/dom/xul/test/test_import_xul_to_content.xhtml b/dom/xul/test/test_import_xul_to_content.xhtml
new file mode 100644
index 0000000000..32db06e564
--- /dev/null
+++ b/dom/xul/test/test_import_xul_to_content.xhtml
@@ -0,0 +1,68 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<window title="Mozilla Importing XUL into Content"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+
+  <!-- test results are displayed in the html:body -->
+  <body xmlns="http://www.w3.org/1999/xhtml">
+  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1027299"
+     target="_blank">Mozilla Bug 1027299</a>
+  </body>
+
+  <browser id="browserelt" src="about:blank" type="content"/>
+
+  <!-- test code goes here -->
+  <script type="application/javascript">
+  <![CDATA[
+
+  SimpleTest.waitForExplicitFinish();
+
+  function expectWarning(expected, when, f) {
+    Services.console.reset();
+
+    f();
+
+    var sawWarning = false;
+    var msgs = Services.console.getMessageArray();
+    for (var i = 0; i < msgs.length; i++) {
+      var msg = msgs[i];
+      if (!msg || !(msg instanceof Ci.nsIScriptError)) {
+        continue;
+      }
+
+      if (msg.category.includes("DOM") && msg.errorMessage.includes("Importing XUL")) {
+        sawWarning = true;
+      }
+    }
+
+    ok(sawWarning == expected, "correct warning condition when " + when);
+  }
+
+  var browser = document.getElementById("browserelt");
+  browser.addEventListener("load", function() {
+    var doc = browser.contentDocument;
+
+    // We add a <video> element, which contains anonymous XUL content. This should not warn.
+    var video = doc.createElement("video");
+    expectWarning(false, "appending video", function() {
+      doc.documentElement.appendChild(video);
+      // Force a layout flush to make sure the anonymous content is added.
+      // eslint-disable-next-line no-unused-vars
+      let dummy = doc.documentElement.offsetLeft;
+    });
+
+    // We add some XUL to a content document. This should generate a warning.
+    var elt = document.createXULElement("scrollbar");
+    var newElt = doc.importNode(elt, false);
+    expectWarning(true, "appending XUL", function() {
+      doc.documentElement.appendChild(newElt);
+    });
+
+    SimpleTest.finish();
+  });
+
+  ]]>
+  </script>
+</window>
diff --git a/dom/xul/test/test_xul_tabindex_focus.xhtml b/dom/xul/test/test_xul_tabindex_focus.xhtml
new file mode 100644
index 0000000000..1363d9b1e1
--- /dev/null
+++ b/dom/xul/test/test_xul_tabindex_focus.xhtml
@@ -0,0 +1,44 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1128054
+-->
+<window title="Mozilla Bug 1128054"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+<body>
+<!-- Test default focusability -->
+<label></label>
+<!-- Test tabindex=0 focusability -->
+<label tabindex="0"></label>
+<!-- Test tabindex=-1 focusability -->
+<label tabindex="-1"></label>
+<!-- Test tabindex=invalid focusability -->
+<label tabindex="invalid"></label>
+<!-- Tests code -->
+<script type="application/javascript">
+<![CDATA[
+
+/** Test for Bug 1128054 **/
+
+add_task(function test_xul_tabindex_focus() {
+  for (let element of document.querySelectorAll("label")) {
+    let desc = "xul element";
+    let focusable = false;
+    if (element.hasAttribute("tabindex")) {
+      let attr = element.getAttribute("tabindex");
+      focusable = Number.isInteger(Number.parseInt(attr));
+      desc += ` with tabindex=${attr}`;
+    }
+
+    element.focus();
+    focusable ? is(document.activeElement, element, desc + " should focusable")
+              : isnot(document.activeElement, element, desc + " should not focusable");
+  }
+});
+
+]]>
+</script>
+</body>
+</window>
diff --git a/dom/xul/test/window_bug1686822.xhtml b/dom/xul/test/window_bug1686822.xhtml
new file mode 100644
index 0000000000..1a2cf25636
--- /dev/null
+++ b/dom/xul/test/window_bug1686822.xhtml
@@ -0,0 +1,7 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:html="http://www.w3.org/1999/xhtml">
+  <html:script type="module"><![CDATA[
+    window.opener.moduleScriptRan();
+  ]]></html:script>
+</window>
diff --git a/dom/xul/test/window_bug583948.xhtml b/dom/xul/test/window_bug583948.xhtml
new file mode 100644
index 0000000000..d0f6a26926
--- /dev/null
+++ b/dom/xul/test/window_bug583948.xhtml
@@ -0,0 +1,8 @@
+<?xml-stylesheet href="chrome://browser/skin/" type="text/css"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" onload="opener.update()">
+
+<command oncommandupdate="document.removeChild(document.documentElement)" commandupdater="true"/>
+<box command="c"/>
+<iframe/>
+
+</window>