I've had this issue for a while. Maybe as long as a year, or longer. Not really sure when it started. It could be that I've had it ever since I began using Grease monkey as I cannot recall when that was. Previously I usually just ignored it and
stopped watching the video if it did not go away after a couple page refreshes. This time I decided to try to track it down. I found someone else who had been having the same
problem and the reply that solved it for them.
When I restarted Firefox in safe mode the problem went away. So I restarted Firefox so all addon's were re-enabled and went through disabling 1 at a time. The problem went away when I disabled Grease monkey. Thats why I'm here.
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The Problem
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Sometimes when I play a YouTube video it'll start and then a second or two later the audio starts playing again. So you basically have 2 audio's playing at the same time. If I pause the video one of the audio's stops(pauses) but the second one keeps playing. When I later unpause the video it's audio starts at where it should be from when I paused it. But the one that didn't pause when the video did just keeps on going.
Is this a known problem and if so is there a solution that doesn't require me to disable Grease monkey?