--- a/dom/base/ScreenOrientation.cpp
+++ b/dom/base/ScreenOrientation.cpp
@@ -363,18 +363,21 @@ ScreenOrientation::LockDeviceOrientation
     return false;
   }
 
   if (NS_WARN_IF(!hal::LockScreenOrientation(aOrientation))) {
     return false;
   }
 
   // We are fullscreen and lock has been accepted.
-  if (aIsFullScreen && !mFullScreenListener) {
-    mFullScreenListener = new FullScreenEventListener();
+  if (aIsFullScreen) {
+    if (!mFullScreenListener) {
+      mFullScreenListener = new FullScreenEventListener();
+    }
+
     aRv = target->AddSystemEventListener(NS_LITERAL_STRING("fullscreenchange"),
                                          mFullScreenListener, /* useCapture = */ true);
     if (NS_WARN_IF(aRv.Failed())) {
       return false;
     }
   }
 
   return true;