--- a/dom/plugins/ipc/PluginModuleChild.cpp
+++ b/dom/plugins/ipc/PluginModuleChild.cpp
@@ -1171,16 +1171,21 @@ NPError
                void* aNotifyData)
 {
     PLUGIN_LOG_DEBUG_FUNCTION;
     ENSURE_PLUGIN_THREAD(NPERR_INVALID_PARAM);
 
     if (!aBuffer)
         return NPERR_INVALID_PARAM;
 
+    if (aIsFile) {
+      PLUGIN_LOG_DEBUG(("NPN_PostURLNotify with file=true is no longer supported"));
+      return NPERR_GENERIC_ERROR;
+    }
+
     nsCString url = NullableString(aRelativeURL);
     auto* sn = new StreamNotifyChild(url);
 
     NPError err;
     InstCast(aNPP)->CallPStreamNotifyConstructor(
         sn, url, NullableString(aTarget), true,
         nsCString(aBuffer, aLength), aIsFile, &err);