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.
Download APP2FIRE from Googleplay app
Download VIKI apk
Procedure To Install
Enable your FIRESTICK Developer Mode and then open your App2Fire Aok and search your firestick IP address
Click on Fire Stick IP, If you are unable to Search IP, Then restart your Firestick, Router and Mobile once.
Click On upload Icon
Click on the Back option and go to the download page
Click on the Download folder
Select app Downloaded in your download folder
Click on APP, then install option will popup click on install and wait.
Thanks, For any support, email me or Ping on Facebook