--- a/editor/libeditor/TextEditRules.cpp
+++ b/editor/libeditor/TextEditRules.cpp
@@ -1061,19 +1061,17 @@ TextEditRules::WillOutputText(Selection*
   if (!aOutString || !aOutputFormat || !aCancel || !aHandled) {
     return NS_ERROR_NULL_POINTER;
   }
 
   // initialize out param
   *aCancel = false;
   *aHandled = false;
 
-  nsAutoString outputFormat(*aOutputFormat);
-  ToLowerCase(outputFormat);
-  if (outputFormat.EqualsLiteral("text/plain")) {
+  if (aOutputFormat->LowerCaseEqualsLiteral("text/plain")) {
     // Only use these rules for plain text output.
     if (IsPasswordEditor()) {
       *aOutString = mPasswordText;
       *aHandled = true;
     } else if (mBogusNode) {
       // This means there's no content, so output null string.
       aOutString->Truncate();
       *aHandled = true;