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YouTube is reportedly slowing down videos for Firefox users (Update: Statement)
(www.androidauthority.com)
Social Experiment. Become Me. What I see, you see.
I've seen it commented that the code itself seems to be an attempt to wait for the ads to show themselves, so it could be just be bad code. But YouTube has already been far more explicitly about their change in policy by interrupting and show warnings to users using adblocks.
I suspect they will eventually move the war over to the served video stream itself, where the browser code will only become explicitly aware of an ad when the minimum necessary for a skip has been played.