--- a/dom/audiochannel/AudioChannelService.cpp
+++ b/dom/audiochannel/AudioChannelService.cpp
@@ -135,17 +135,17 @@ AudioChannelService::UnregisterType(Audi
   // In order to avoid race conditions, it's safer to notify any existing
   // agent any time a new one is registered.
   if (XRE_GetProcessType() == GeckoProcessType_Default) {
     // We only remove ChildID when it is in the foreground.
     // If in the background, we kept ChildID for allowing it to play next song.
     if (aType == AUDIO_CHANNEL_CONTENT &&
         mActiveContentChildIDs.Contains(aChildID) &&
         (!mActiveContentChildIDsFrozen &&
-          mChannelCounters[AUDIO_CHANNEL_INT_CONTENT].IndexOf(aChildID) == -1)) {
+          !mChannelCounters[AUDIO_CHANNEL_INT_CONTENT].Contains(aChildID))) {
       mActiveContentChildIDs.RemoveElement(aChildID);
     }
     SendAudioChannelChangedNotification();
     Notify();
   }
 }
 
 bool
@@ -298,18 +298,18 @@ AudioChannelService::SendAudioChannelCha
     else if (!mChannelCounters[AUDIO_CHANNEL_INT_NOTIFICATION_HIDDEN].IsEmpty()) {
       higher = AUDIO_CHANNEL_NOTIFICATION;
     }
 
     // There is only one Child can play content channel in the background.
     // And need to check whether there is any content channels under playing
     // now.
     else if (!mActiveContentChildIDs.IsEmpty() &&
-             mChannelCounters[AUDIO_CHANNEL_INT_CONTENT_HIDDEN].IndexOf(
-             mActiveContentChildIDs[0]) != -1) {
+             mChannelCounters[AUDIO_CHANNEL_INT_CONTENT_HIDDEN].Contains(
+             mActiveContentChildIDs[0])) {
       higher = AUDIO_CHANNEL_CONTENT;
     }
   }
 
   if (higher != mCurrentHigherChannel) {
     mCurrentHigherChannel = higher;
 
     nsString channelName;