I would like to report a possible bug. When you set the media.autoplay.default in about:config to value of 2 and allow video autoplay on certain webpages, then when you try to adjust the volume, the video pauses. It is currently happening on two of my PCs (both on W7 64-bit) on various sites like youtube, twitch etc. What I had to do to fix it was to change the value of media.autoplay.default back to 1 and then remove the permission in the menu on the left of the address line. What I would further suggest is not to display those annoying popup dialogues for user to allow autoplay on each webpage, but just put a switch for that into the menu in the address line (or do both ways and make it possible to turn those dialogues off).