In this case themes and extensions need to check. This makes us suspicious about the themes and extensions being used. But many users are facing YouTube playback issues while using Firefox.
This may be related to flash players, plugins, and HTML5 Video players and maybe because of the corrupt extension and themes. Like while watching any video suddenly it stops working.
On the other hand, if one will use other browsers like Google chrome, Daily motion, etc. These kinds of problems often persist and these are really disturbing ones. Sometimes the video directly goes to the end and the browser or cannot play it properly.