So is that voice command going to an Echo device first and then it should open the request on the Fire TV? Or does the voice command go direct to the Fire TV?
If the former do you have to tell Alexa which device to open that thing on? E.G. "Alexa play The Man in the High Castle on the TV" where TV might be the device name of the Fire TV? Using Plex as an example, I had to tell my Echo Dot to use the Plex app on my TV as the device that Alexa implements voice commands on. I had to first tell Alexa which Plex server to use, then the Plex player (being the Apple TV Plex app). Once done, it now does it by default to that device every time. I wonder if Fire TV playback is similar?
Of course if it was the latter, that paragraph is redundant :)
You can set defaults for some things, like what your default music player is so you don't need to say, for example, "Alexa, player song XYZ on spotify" - you can just set Spotify as default then say "Alexa, play song XYZ" I don't think there is capacity for entertainment system though, should you have multiple devices, so you would have to say "Alexa, play XYZ on the Fire TV" as per:
"Note: You may need to clarify that you'd like to stream on a Fire TV device by adding, "on Fire TV" to the end of your request."