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Make Kodi or any app boot from start up on your Amazon Fire TV Stick
Load AVS Fire Launcher to your Fire TV. Launch the AVS Fire Launcher App on your Amazon Fire TV. Press OK to open up the app options. Select the app you want to launch from boot and press OK. Restart the Fire TV device in Settings > Device > Restart. The Amazon Fire TV device will reboot and launch your chosen application.
When your Amazon Fire TV Stick or Box reboots the Amazon home screen will start to load. It takes a second or two for the launcher to kick in so don't panic. If the user closes the auto launched application they will be able to access the Amazon default launcher so again, no need to panic. You can easily remove the launcher or edit it's launch settings.