Amazon Fire TV 3 (amazon-needle)

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 * BenTheTechGuy

Unlocking the bootloader
Root the device using the FireFU exploit, which takes advantage of vulnerabilities in Amlogic DFU mode and U-Boot to unlock the bootloader and disable dm-verity. You will need an I2C-capable Arduino such as the Due as well as an HDMI breakout board (or a cut HDMI cable).

NOTE: The FireFU exploit does not work on devices running FireOS 6.2.5.8 or newer. Even after unlocking the bootloader, upgrading to those versions of FireOS disables flashing of unsigned images in Fastboot. Due to FireFU's repositioning of the boot image's offsets, your device will be bricked if it upgrades past 6.2.5.5 with FireFU installed. It's highly recommended to block all OTA updates after rooting the device.

How to enter flash mode
In FireOS, connect with adb over the network or USB and run.

In DFU mode, run  in the FireFU directory.