User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.102 Safari/537.36 Vivaldi/1.93.955.42 Steps to reproduce: Pin (or open sometimes and wait to add automatically in the Top Sites) video/stream like twitch ( https://go.twitch.tv/videos/{number} ) Actual results: Firefox open the site and "play" (in the background silently) the video with unreasonably high CPU and memory usage and it does not stop, eat all the memory. Expected results: Generate page screenshot without play the media content.