[Fire Q&A, Help & Troubleshooting] obsidience:
I played around with fastboot a bit and have included the logs below. What's interesting is that formating userdata and cache fails with a "Formatting is not supported for filesystem with type 'unknown'.".

Leads me to believe that the filesystem is somehow corrupt. Not an expert though.

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot.exe boot ..\apps\stock-recovery.img

downloading 'boot.img'...

OKAY [ 1.051s]

booting...

FAILED (remote: unknown command)

finished. total time: 1.058s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot flash recovery ..\apps\stock-recovery.img

target reported max download size of 134217728 bytes

sending 'recovery' (16384 KB)...

OKAY [ 1.052s]

writing 'recovery'...

FAILED (remote: Flashing not allowed)

finished. total time: 1.063s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot format userdata

Formatting is not supported for filesystem with type 'unknown'.

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem unlock

...

FAILED (remote: unknown command)

finished. total time: 0.005s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot format cache

Formatting is not supported for filesystem with type 'unknown'.

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot flash recovery ..\apps\TWRP_Fire_2.8.7.0.img

target reported max download size of 134217728 bytes

sending 'recovery' (7724 KB)...

OKAY [ 0.505s]

writing 'recovery'...

FAILED (remote: Flashing not allowed)

finished. total time: 0.516s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot.exe boot ..\apps\stock-recovery.img

downloading 'boot.img'...

OKAY [ 1.051s]

booting...

FAILED (remote: unknown command)

finished. total time: 1.057s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot.exe boot ..\apps\TWRP_Fire_2.8.7.0.img

downloading 'boot.img'...

OKAY [ 0.505s]

booting...

FAILED (remote: unknown command)

finished. total time: 0.511s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot.exe boot ..\apps\cm-12-recovery.img

downloading 'boot.img'...

OKAY [ 0.459s]

booting...

FAILED (remote: unknown command)

finished. total time: 0.465s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar version

version: 0.5

finished. total time: 0.003s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar version-bootloader

version-bootloader:

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar version-cpld

version-cpld:

finished. total time: 0.003s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar partition-layout

partition-layout:

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar security

security:

finished. total time: 0.003s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar build-mode

build-mode:

finished. total time: 0.003s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot getvar boot-mode

boot-mode:

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem keytest

...

FAILED (remote: unknown command)

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem heap

...

FAILED (remote: unknown command)

finished. total time: 0.005s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem rebootRUU

...

FAILED (remote: unknown command)

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem task

...

FAILED (remote: unknown command)

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot oem rtask

...

FAILED (remote: unknown command)

finished. total time: 0.004s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot reboot-bootloader

rebooting into bootloader...

OKAY [ 0.004s]

finished. total time: 0.005s

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>fastboot devices

G0K0H404539208VF fastboot

C:\Users\User\Downloads\Amazon Fire\AmazonFire5thGenSuperTool\files>