11/15/19: I had a simililar Xfinity stream issue with Firefox version 70.0.1 64 bit. Reading some of the feedback here, i found out i had an add in, under extensions for "Ghostery - Privacy Ad Blocer" that i had installed some time ago on my desktop. My laptop doesn't have this add on and xfinity Firefox streaming works, soooooooooo. When i disabled the "Ghostery" add on, on my desktop, Firefox can now stream Xfinity movies!!!!
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