The addition of the free file browser, Total Commander, to the official Fire TV appstore. Now makes it possible to sideload apps like Kodi onto the Fire TV without the need to use ADB and a PC.
As long as you can load a USB drive with an APK file, you can sideload it onto the Fire TV without any outside connection or utility.
This method uses an app called Total Commander (installable directly from the Fire TV Appstore) to install APK files stored on a USB drive. This method works for the 1st-gen and 2nd-gen Fire TV, but does not work for the Fire TV Stick because it does not have a standard USB port. You only need a computer to transfer the APK files you want installed onto the USB drive. Then you can do everything else directly on the Fire TV without the need of a computer or ADB. This method is ideal if you want to install many APKs at once or very large APKs since you don’t have to wait for each one to be transferred to the Fire TV.