It's not just your FireTV. It's also an issue with Roku. Comcast is one ISP that mainly wants you to use a cable box to watch TV on a real TV. They often only allow apps on mobile devices.
There is a workaround for Roku that might work with FireTV. It works for my Roku and the Travel Channel, which is part of the HGTV family of networks:
Comcast is still stuck in the '90s. They have improved somewhat with the X1 system but they are fighting tooth & nail when it comes to authorizing TV Everywhere apps on devices such as the Fire TV, Roku, Android TV & Apple TV. Their excuse is always that it's a business decision. Now that there are more options becoming available for streaming TV such as Sling TV, PS Vue, Directv Now, Youtube Unplugged & Hulu's upcoming tv service maybe Comcast will adjust with the current state of the industry or continue to lose customers to competing services.
Seems to me, that Comcast is not listed with many streaming apps because they are preventing people from cutting the cord. I do hope that if I get Hulu TV or any of the others, that I can watch those channels they claim to have without a "provider's auhorization" like we do for Fire TV apps! If so, we will have to over pay Comcast for TV forever.
smh @ Comcast.