diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 3d17e502a10e..69e9dee5ff09 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -5818,12 +5818,12 @@ nsGlobalWindow::GetChildWindow(const nsAString& aName)
 }
 
 bool
-nsGlobalWindow::DispatchCustomEvent(const char *aEventName)
+nsGlobalWindow::DispatchCustomEvent(const nsAString& aEventName)
 {
+  MOZ_ASSERT(IsOuterWindow());
+
   bool defaultActionEnabled = true;
-  nsContentUtils::DispatchTrustedEvent(mDoc,
-                                       GetOuterWindow(),
-                                       NS_ConvertASCIItoUTF16(aEventName),
+  nsContentUtils::DispatchTrustedEvent(mDoc, ToSupports(this), aEventName,
                                        true, true, &defaultActionEnabled);
 
   return defaultActionEnabled;
@@ -5999,7 +5999,7 @@ nsGlobalWindow::SetFullScreenInternal(bool aFullScreen, bool aRequireTrust)
 
   // dispatch a "fullscreen" DOM event so that XUL apps can
   // respond visually if we are kicked into full screen mode
-  if (!DispatchCustomEvent("fullscreen")) {
+  if (!DispatchCustomEvent(NS_LITERAL_STRING("fullscreen"))) {
     return NS_OK;
   }
 
@@ -8435,7 +8435,7 @@ nsGlobalWindow::Close(ErrorResult& aError)
   bool wasInClose = mInClose;
   mInClose = true;
 
-  if (!DispatchCustomEvent("DOMWindowClose")) {
+  if (!DispatchCustomEvent(NS_LITERAL_STRING("DOMWindowClose"))) {
     // Someone chose to prevent the default action for this event, if
     // so, let's not close this window after all...
 
@@ -8474,7 +8474,7 @@ nsGlobalWindow::ForceClose()
 
   mInClose = true;
 
-  DispatchCustomEvent("DOMWindowClose");
+  DispatchCustomEvent(NS_LITERAL_STRING("DOMWindowClose"));
 
   FinalClose();
 }
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index 091e0324f30e..8713d7cf35ab 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -487,9 +487,9 @@ public:
 
   virtual void MaybeUpdateTouchState();
   virtual void UpdateTouchState();
-  virtual bool DispatchCustomEvent(const char *aEventName);
 
   // Outer windows only.
+  virtual bool DispatchCustomEvent(const nsAString& aEventName);
   bool DispatchResizeEvent(const mozilla::CSSIntSize& aSize);
 
   virtual void RefreshCompartmentPrincipal();
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index f06cf5a715b6..b1819f7b8501 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -677,8 +677,10 @@ public:
   /**
    * Dispatch a custom event with name aEventName targeted at this window.
    * Returns whether the default action should be performed.
+   *
+   * Outer windows only.
    */
-  virtual bool DispatchCustomEvent(const char *aEventName) = 0;
+  virtual bool DispatchCustomEvent(const nsAString& aEventName) = 0;
 
   /**
    * Notify the active inner window that the document principal may have changed
diff --git a/toolkit/components/alerts/nsXULAlerts.cpp b/toolkit/components/alerts/nsXULAlerts.cpp
index 3d3b685ce057..b54defaa8013 100644
--- a/toolkit/components/alerts/nsXULAlerts.cpp
+++ b/toolkit/components/alerts/nsXULAlerts.cpp
@@ -152,7 +152,8 @@ nsXULAlerts::CloseAlert(const nsAString& aAlertName)
   nsIDOMWindow* alert = mNamedWindows.GetWeak(aAlertName);
   nsCOMPtr<nsPIDOMWindow> domWindow = do_QueryInterface(alert);
   if (domWindow) {
-    domWindow->DispatchCustomEvent("XULAlertClose");
+    MOZ_ASSERT(domWindow->IsOuterWindow());
+    domWindow->DispatchCustomEvent(NS_LITERAL_STRING("XULAlertClose"));
   }
   return NS_OK;
 }
diff --git a/xpfe/appshell/src/nsWebShellWindow.cpp b/xpfe/appshell/src/nsWebShellWindow.cpp
index 35036544e7ff..940f50d49854 100644
--- a/xpfe/appshell/src/nsWebShellWindow.cpp
+++ b/xpfe/appshell/src/nsWebShellWindow.cpp
@@ -330,6 +330,8 @@ nsWebShellWindow::SizeModeChanged(nsSizeMode sizeMode)
   nsCOMPtr<nsPIDOMWindow> ourWindow =
     mDocShell ? mDocShell->GetWindow() : nullptr;
   if (ourWindow) {
+    MOZ_ASSERT(ourWindow->IsOuterWindow());
+
     // Let the application know if it's in fullscreen mode so it
     // can update its UI.
     if (sizeMode == nsSizeMode_Fullscreen) {
@@ -340,7 +342,7 @@ nsWebShellWindow::SizeModeChanged(nsSizeMode sizeMode)
     }
 
     // And always fire a user-defined sizemodechange event on the window
-    ourWindow->DispatchCustomEvent("sizemodechange");
+    ourWindow->DispatchCustomEvent(NS_LITERAL_STRING("sizemodechange"));
   }
 
   // Note the current implementation of SetSizeMode just stores