I had the same issue on mine. Heres the fix:
In TWRP with the fire connected to the pc run:
echo /dev/block/mmcblk0p12 > /sys/devices/platform/usb_mass_storage/lun0/file
Windows will probably respond that the volume/device needs formatted before using it.
Thats okay. choose quick format and let it format it as fat32 block size 4096.
when its done
in twrp choose the mount option and mount USB storage then hit unmount again
tap the box next to Mount SDCard. It should now look like
[X] Unmount SDCard.
Download the 6.2.1 pre-rooted zip file from the thread in the dev section.
choose mount USB Storage
Place on SDCard
Turn off usb storage
choose factory reset
choose dalvik cache
go back and choose install
choose the zip you placed on your sdcard
reboot. if it reboots back into twrp
idme ? <--that will probably tell you the bootmode is 0x5003
idme bootmode 0x4000
in twrp choose reboot
enjoy! it may take a while to boot