--- a/dom/media/DecoderDoctorDiagnostics.cpp
+++ b/dom/media/DecoderDoctorDiagnostics.cpp
@@ -368,17 +368,17 @@ DecoderDoctorDocumentWatcher::Synthesize
 #endif
   nsAutoString playableFormats;
   nsAutoString unplayableFormats;
   nsAutoString supportedKeySystems;
   nsAutoString unsupportedKeySystems;
   DecoderDoctorDiagnostics::KeySystemIssue lastKeySystemIssue =
     DecoderDoctorDiagnostics::eUnset;
 
-  for (auto& diag : mDiagnosticsSequence) {
+  for (const auto& diag : mDiagnosticsSequence) {
     switch (diag.mDecoderDoctorDiagnostics.Type()) {
       case DecoderDoctorDiagnostics::eFormatSupportCheck:
         if (diag.mDecoderDoctorDiagnostics.CanPlay()) {
           canPlay = true;
           AppendToStringList(playableFormats,
                              diag.mDecoderDoctorDiagnostics.Format());
         } else {
 #if defined(XP_WIN)