diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 9380d4043505..bfadc33ea2ad 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -10479,6 +10479,44 @@ nsIFrame::IsScrolledOutOfView()
   return IsFrameScrolledOutOfView(this);
 }
 
+static already_AddRefed<nsIWidget>
+GetWindowWidget(nsPresContext* aPresContext)
+{
+  // We want to obtain the widget for the window. We can't use any of these
+  // methods: nsPresContext::GetRootWidget, nsPresContext::GetNearestWidget,
+  // nsIFrame::GetNearestWidget because those deal with child widgets and
+  // there is no parent widget connection between child widgets and the
+  // window widget that contains them.
+  nsCOMPtr<nsISupports> container = aPresContext->Document()->GetContainer();
+  nsCOMPtr<nsIBaseWindow> baseWindow = do_QueryInterface(container);
+  if (!baseWindow) {
+    return nullptr;
+  }
+
+  nsCOMPtr<nsIWidget> mainWidget;
+  baseWindow->GetMainWidget(getter_AddRefs(mainWidget));
+  return mainWidget.forget();
+}
+
+void
+nsIFrame::UpdateWidgetProperties()
+{
+  nsPresContext* presContext = PresContext();
+  if (presContext->IsRoot() || !presContext->IsChrome()) {
+    // Don't do anything for documents that aren't the root chrome document.
+    return;
+  }
+  nsIFrame* rootFrame =
+    presContext->FrameConstructor()->GetRootElementStyleFrame();
+  if (this != rootFrame) {
+    // Only the window's root style frame is relevant for widget properties.
+    return;
+  }
+  if (nsCOMPtr<nsIWidget> widget = GetWindowWidget(presContext)) {
+    widget->SetWindowOpacity(StyleUIReset()->mWindowOpacity);
+  }
+}
+
 void
 nsIFrame::DoUpdateStyleOfOwnedAnonBoxes(ServoStyleSet& aStyleSet,
                                         nsStyleChangeList& aChangeList,
diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index 0c30fbea9fce..73b8b8d6564e 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -3934,7 +3934,10 @@ public:
    */
   bool IsScrolledOutOfView();
 
-  virtual void UpdateWidgetProperties() {}
+  /**
+   * Applies the values from the -moz-window-* properties to the widget.
+   */
+  virtual void UpdateWidgetProperties();
 
   /**
    * @return true iff this frame has one or more associated image requests.
diff --git a/layout/xul/nsMenuPopupFrame.cpp b/layout/xul/nsMenuPopupFrame.cpp
index 1a6cb5d667bf..cabfe5ab771a 100644
--- a/layout/xul/nsMenuPopupFrame.cpp
+++ b/layout/xul/nsMenuPopupFrame.cpp
@@ -456,6 +456,14 @@ nsMenuPopupFrame::IsLeafDynamic() const
           !parentContent->HasAttr(kNameSpaceID_None, nsGkAtoms::sizetopopup));
 }
 
+void
+nsMenuPopupFrame::UpdateWidgetProperties()
+{
+  if (nsIWidget* widget = GetWidget()) {
+    widget->SetWindowOpacity(StyleUIReset()->mWindowOpacity);
+  }
+}
+
 void
 nsMenuPopupFrame::LayoutPopup(nsBoxLayoutState& aState, nsIFrame* aParentMenu,
                               nsIFrame* aAnchor, bool aSizedToPopup)
diff --git a/layout/xul/nsMenuPopupFrame.h b/layout/xul/nsMenuPopupFrame.h
index 6df5e2342933..25b893e24ba8 100644
--- a/layout/xul/nsMenuPopupFrame.h
+++ b/layout/xul/nsMenuPopupFrame.h
@@ -248,6 +248,8 @@ public:
 
   virtual bool IsLeafDynamic() const override;
 
+  virtual void UpdateWidgetProperties() override;
+
   // layout, position and display the popup as needed
   void LayoutPopup(nsBoxLayoutState& aState, nsIFrame* aParentMenu,
                    nsIFrame* aAnchor, bool aSizedToPopup);