I am writing this post because I also came in across with below help centre of viki.com that fire tv will not support the Viki app which OS is 5.2.8 or less. So VIKI apk for FireTV OS I have apk.
So I have searched many other APK and found one compatible VIKI apk for FireTV.
You can install this app from your android mobile through APP2FIRE VIKI apk for FireTV.
Check Viki Help Center – Click Here