I'm pretty sure it's the firestick. I don't have issues on any of my other devices (phone, ipad, laptop)....of course, those are all apple products. But I think it works on the kindle (which is android, right?).
I asked about your provider, because I have heard things about MLB.tv and Uverse costumers. But I have Uverse, and like I said, it works fine on all my other devices...just not the firestick.
If you get anything solved, post back.