I just noticed this tonight... not sure if it's my setup or if YouTube is actively doing something. While watching a video on YouTube (Firefox latest on Mavericks, mid-2012 MBP), I click on AirPlay to send the screen to my ATV. My display mirrors fine, but the visual part of the video stops on both the MBP and the TV. A couple seconds later, the audio stops. When I turn mirroring off, the video starts right back up again. Just me?