Thank you so much for posting your question in the Xfinity Forum. At this time, the Xfinity Stream app is only compatible with the following Amazon devices:
I would recommend either coming back to the forum later for updates to this or see here: https://www.xfinity.com/support/xfinity-apps/xfinity-tv-app-requirements/#Amazon
So if I'm reading this list correctly, you only support Fire Tablets, but not Fire TV? That's crazy.
When will you come out with an app for the Firestick? I'm trying to ditch some of my equipment and this would be a perfect solution for a few of my TVs. I already own Firesticks and don't want to switch to Roku.