diff --git a/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp b/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp
index 6f0f09a73a1..9c939cdc7fb 100644
--- a/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1976,6 +1976,9 @@ nsresult nsExternalAppHandler::OpenWithApplication()
   // if a stop request was already issued then proceed with launching the application.
   if (mStopRequestIssued)
   {
+    // make the tmp file readonly so users won't edit it and lose the changes
+    mFinalFileDestination->SetPermissions(0400);
+
     rv = mMimeInfo->LaunchWithFile(mFinalFileDestination);        
     if (NS_FAILED(rv))
     {