Follow the simple steps below to enable ADB on your Fire TV device.
Note: This guide is for Fire TV OS versions older than 126.96.36.199 See this guide for newer OS versions.
While on the home screen scroll down to Settings.
Scroll to the right then select System.
With the System dialog open select Developer Options.
Turn ADB debugging and Apps from Unknown Sources ON.
ADB Debugging is now enabled on your Fire TV or Fire TV Stick. Your Fire TV is now setup to allow side-loading .apk files.