links:

[Installing ADB and Fastboot on Linux & Device Detection "Drivers"](https://forum.xda-developers.com/android/general/guide-installing-adb-fastboot-linux-adb-t3478678)

You need to be on 6.3.1.2 firmware:
[Fire HD 8 2018 (karnak) amonet-3](https://forum.xda-developers.com/hd8-hd10/orig-development/unlock-fire-hd-8-2018-karnak-amonet-3-t3963496/page52)

[Download 6.3.1.2 firmware](https://fireos-tablet-src.s3.amazonaws.com/LlO8A9g4Q6ugQCylaeqWBWxYBb/update-kindle-Fire_HD8_8th_Gen-NS6312_user_1852_0002517056644.bin)

2. Download the amazon frimware above and keep it where you can flash it.
3. Boot into recovery (Volume Down + Power at the same time)
4. Select "adb sideload" or whatever it says using your volume keys and press the power to select
5. Now adb sideload <frimware>.bin

https://www.youtube.com/watch?v=sN6PphcI6XQ