Finally found an add-on that can disable auto-play in playlists. Just go to preferences from the add-on page and select 'Stop auto-buffering the player on play-list mode'

To prevent it from playing a few seconds of video, you can go to about:config and set "media.autoplay.enabled" to false. This prevents unwatched videos from showing up in your watch history. However, it technically still buffers for a couple seconds before refreshing. This firefox setting does not prevent auto-play in playlists, which is why I downloaded this plugin.