--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -1992,17 +1992,19 @@ XMLHttpRequestMainThread::OnStartRequest
             }
           }
           nsCOMPtr<nsIFile> jarFile;
           jarChannel->GetJarFile(getter_AddRefs(jarFile));
           if (!jarFile) {
             mIsMappedArrayBuffer = false;
           } else {
             rv = mArrayBufferBuilder.mapToFileInPackage(file, jarFile);
-            if (NS_WARN_IF(NS_FAILED(rv))) {
+            // This can happen legitimately if there are compressed files
+            // in the jarFile. See bug #1357219. No need to warn on the error.
+            if (NS_FAILED(rv)) {
               mIsMappedArrayBuffer = false;
             } else {
               channel->SetContentType(NS_LITERAL_CSTRING("application/mem-mapped"));
             }
           }
         }
       }
     }