--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -3091,27 +3091,29 @@ nsDOMWindowUtils::HandleFullscreenReques
   return NS_OK;
 }
 
 nsresult
 nsDOMWindowUtils::ExitFullscreen()
 {
   nsCOMPtr<nsIDocument> doc = GetDocument();
   NS_ENSURE_STATE(doc);
+
+  // Although we would not use the old size if we have already exited
+  // fullscreen, we still want to cleanup in case we haven't.
+  nsSize oldSize = OldWindowSize::GetAndRemove(doc->GetWindow());
   if (!doc->IsFullScreenDoc()) {
     return NS_OK;
   }
 
   // Notify the pres shell that we are starting fullscreen change, and
   // set the window dimensions in advance. Since the resize message
   // comes after the fullscreen change call, doing so could avoid an
   // extra resize reflow after this point.
-  FullscreenChangePrepare prepare(
-    GetPresShell(), OldWindowSize::GetAndRemove(doc->GetWindow()));
-
+  FullscreenChangePrepare prepare(GetPresShell(), oldSize);
   nsIDocument::ExitFullscreenInDocTree(doc);
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMWindowUtils::SelectAtPoint(float aX, float aY, uint32_t aSelectBehavior,
                                 bool *_retval)
 {