Usually all of the formats EXCEPT H.264/MP4 can play. YouTube has a page to check that here: https://www.youtube.com/html5 If you do not at least have blue for HTMLVideoElement and WebM VP8, we'll need to have you check some settings. ---- (copy/paste) ''If you can play older videos but not new ones:'' According to other users, YouTube recently dropped Flash formats, and now only uses an HTML5 video player. Firefox users on Windows XP are experiencing delays in when they can play new videos. My theory on this is that the creator uploaded the video in a format that Firefox cannot decode on Windows XP (H.264/MP4) because Firefox requires the Microsoft Media Foundation and Microsoft only released it for Vista and later versions. However, Google transcodes videos to other formats in the background, so eventually a playable format becomes available. There is a potential workaround for Firefox 52 Extended Support Release on Windows XP here (I haven't tried this myself): http://www.msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/