Every time I'm on youtube the sound cuts out.

Hi. Every time I go into youtube, the sound cuts out after about an hour. I then have to go to settings, volume mixer and Mozilla Firefox and click on the X to get the sound back. Youtube has assured me this is not their problem, but a Mozilla one. I've been with Firefox for many many years and have never had this problem. Could you please advise me as to what to do about this ? Thank you.