Serenity for Android should now be available on the Amazon App store for the Amazon Fire TV. You may have to search for it by name to find it. I've submitted a newer version of 1.7.3 that contains all the fixes that are available in Google Play as well as on the project's download site.
The version is functionally equivalent and everything that works in the Tablet and Android TV version works on the Amazon Fire TV. They are the exact same app. Unfortunately if you purchased it on Google Play you will need to repurchase it on Amazon. Not much I can do about that. Price remains the same, no price drop. If you want an alternative to the official app, you can give Serenity a try for less than a Starbucks coffee.