--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -2239,16 +2239,17 @@ TabParent::RecvAccessKeyNotHandled(const
 
   // This is called only when this process had focus and HandleAccessKey
   // message was posted to all remote process and each remote process didn't
   // execute any content access keys.
   // XXX If there were two or more remote processes, this may be called
   //     twice or more for a keyboard event, that must be a bug.  But how to
   //     detect if received event has already been handled?
 
+  MOZ_ASSERT(aEvent.mMessage == eKeyPress);
   WidgetKeyboardEvent localEvent(aEvent);
   localEvent.MarkAsHandledInRemoteProcess();
   localEvent.mMessage = eAccessKeyNotFound;
 
   // Here we convert the WidgetEvent that we received to an nsIDOMEvent
   // to be able to dispatch it to the <browser> element as the target element.
   nsIDocument* doc = mFrameElement->OwnerDoc();
   nsIPresShell* presShell = doc->GetShell();

--- a/dom/xbl/nsXBLWindowKeyHandler.cpp
+++ b/dom/xbl/nsXBLWindowKeyHandler.cpp
@@ -351,16 +351,22 @@ nsXBLWindowKeyHandler::InstallKeyboardEv
                            this, NS_LITERAL_STRING("keydown"),
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->AddEventListenerByType(
                            this, NS_LITERAL_STRING("keyup"),
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->AddEventListenerByType(
                            this, NS_LITERAL_STRING("keypress"),
                            TrustedEventsAtSystemGroupBubble());
+  // mozaccesskeynotfound event is fired when modifiers of keypress event
+  // matches with modifier of content access key but it's not consumed by
+  // remote content.
+  aEventListenerManager->AddEventListenerByType(
+                           this, NS_LITERAL_STRING("mozaccesskeynotfound"),
+                           TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->AddEventListenerByType(
                            this, NS_LITERAL_STRING("mozkeydownonplugin"),
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->AddEventListenerByType(
                            this, NS_LITERAL_STRING("mozkeyuponplugin"),
                            TrustedEventsAtSystemGroupBubble());
 }
 
@@ -405,16 +411,19 @@ nsXBLWindowKeyHandler::RemoveKeyboardEve
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->RemoveEventListenerByType(
                            this, NS_LITERAL_STRING("keyup"),
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->RemoveEventListenerByType(
                            this, NS_LITERAL_STRING("keypress"),
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->RemoveEventListenerByType(
+                           this, NS_LITERAL_STRING("mozaccesskeynotfound"),
+                           TrustedEventsAtSystemGroupBubble());
+  aEventListenerManager->RemoveEventListenerByType(
                            this, NS_LITERAL_STRING("mozkeydownonplugin"),
                            TrustedEventsAtSystemGroupBubble());
   aEventListenerManager->RemoveEventListenerByType(
                            this, NS_LITERAL_STRING("mozkeyuponplugin"),
                            TrustedEventsAtSystemGroupBubble());
 }
 
 /* static */ KeyboardMap
@@ -460,17 +469,23 @@ nsXBLWindowKeyHandler::ConvertEventToDOM
                          const WidgetKeyboardEvent& aWidgetKeyboardEvent) const
 {
   if (aWidgetKeyboardEvent.IsKeyDownOrKeyDownOnPlugin()) {
     return nsGkAtoms::keydown;
   }
   if (aWidgetKeyboardEvent.IsKeyUpOrKeyUpOnPlugin()) {
     return nsGkAtoms::keyup;
   }
-  if (aWidgetKeyboardEvent.mMessage == eKeyPress) {
+  // eAccessKeyNotFound event is always created from eKeyPress event and
+  // the original eKeyPress event has stopped its propagation before dispatched
+  // into the DOM tree in this process and not matched with remote content's
+  // access keys.  So, we should treat it as an eKeyPress event and execute
+  // a command if it's registered as a shortcut key.
+  if (aWidgetKeyboardEvent.mMessage == eKeyPress ||
+      aWidgetKeyboardEvent.mMessage == eAccessKeyNotFound) {
     return nsGkAtoms::keypress;
   }
   MOZ_ASSERT_UNREACHABLE(
     "All event messages which this instance listens to should be handled");
   return nullptr;
 }
 
 NS_IMETHODIMP