Are you watching Russian based youtube video or other Regions? Or whomever made it used unrecognized format so it doesn't work.

''Vldmr [[#question-1247688|said]]'' <blockquote> Can't see video from YouTube site. </blockquote> Are you indeed still using the old EOL Windows XP ? This is the reason why you cannot fully use HTML5 players like used by Youtube. Go to https://www.youtube.com/html5/ as you will likely have three red check marks when all should be blue ideally. The problem is Microsoft did not support Windows XP and they do with Vista and later with the media feature packs that codecs the codes needed by HTML5 players. The problem is Microsoft did not support Windows XP as they do with Vista and later with the media feature packs that provided needed codecs used by HTML5 players. This is for Windows Vista and later users but explains what I mean about the packs. https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows You can see the limit of what you support at https://www.youtube.com/html5/ as you likely have three red check marks instead of all being blue. With some work you may be able to support HTML5 players better in Firefox 52 ESR on WinXP. https://msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/ The only way you will be able to fully support HTML5 players and current Firefox versions is to use Windows 7 or later or dual boot with a light 32-bit Linux distro like Lubuntu or Xubuntu among others.