Having 2 Kindle Fire "5th generation"s that are pretty much useless for not getting proper software, I had good hope of putting on a custom ROM after reading this post. But as always with old posts, not all links work any more and I seem to be stuck in the step of downgrading the Fire OS to an older version.
The device is now in fastboot mode, and the batch script that would do the downgrading says that the device cannot be found. Perhaps it should be in recovery for flashing the
.bin file with the older OS. It's fairly easy to come out of this state by using
fastboot reboot but the OS has not changed.
I did try to flash a TWRP recovery image but the hardware is still locked:
fastboot flash recovery TWRP_Fire_188.8.131.52.img Sending 'recovery' (7714 KB) OKAY [ 0.297s] Writing 'recovery' FAILED (remote: 'the command you input is restricted on locked hw') Finished. Total time: 0.313s
Booting into the Amazon 'recovery' is also still possible but I guess installing a custom ROM zipfile from there with ADB will just brick the device?!