--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -1894,18 +1894,20 @@ DocumentViewerImpl::SetBounds(const nsIn
 
   // If there's a previous viewer, it's the one that's actually showing,
   // so be sure to resize it as well so it paints over the right area.
   // This may slow down the performance of the new page load, but resize
   // during load is also probably a relatively unusual condition
   // relating to things being hidden while something is loaded.  It so
   // happens that Firefox does this a good bit with its infobar, and it
   // looks ugly if we don't do this.
-  if (mPreviousViewer)
-    mPreviousViewer->SetBounds(aBounds);
+  if (mPreviousViewer) {
+    nsCOMPtr<nsIContentViewer> previousViewer = mPreviousViewer;
+    previousViewer->SetBounds(aBounds);
+  }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 DocumentViewerImpl::Move(int32_t aX, int32_t aY)
 {
   NS_ENSURE_TRUE(mDocument, NS_ERROR_NOT_AVAILABLE);