[Fire TV Original Android Development] fabian19vs:
I think everything was fine but maybe because of the different boot.img , it is still stuck in the b/w logo boot loop. Maybe there is a way to update to the most recent version of fireOS? when I did the "fastboot format user data" the result shows 18-Aug-2018, so I don't think that the version it has installed is a recent one.

Maybe something is preventing TWRP to launch correctly on boot, because when I get out of the fastboot state, adb is not recognizing the stick

[root@sysresccd ~/Downloads/kamakiri/kamakiri-mantis-v1.2/kamakiri]# sudo ./bootrom-step.sh
[2021-02-18 00:13:55.022025] Waiting for bootrom
[2021-02-18 00:14:35.003077] Found port = /dev/ttyACM0
[2021-02-18 00:14:35.003467] Handshake
[2021-02-18 00:14:35.003997] Disable watchdog

* * * If you have a short attached, remove it now * * *
* * * Press Enter to continue * * *

[2021-02-18 00:14:41.078406] Load payload from ../brom-payload/stage1/stage1.bin = 0x27C bytes
[2021-02-18 00:14:41.252936] Let's rock
[2021-02-18 00:14:41.259575] Waiting for stage 1 to come online...
[2021-02-18 00:14:41.261696] All good
[2021-02-18 00:14:41.261825] Load 2nd stage payload
[2021-02-18 00:14:41.261956] Load payload from ../brom-payload/stage2/stage2.bin = 0x36D4 bytes
[2021-02-18 00:14:41.262052] Send 2nd stage payload
[2021-02-18 00:14:41.286117] Party time
[2021-02-18 00:14:41.287735] Waiting for stage 2 to come online...
[2021-02-18 00:14:41.541332] All good
[2021-02-18 00:14:41.543057] Check GPT
[2021-02-18 00:14:41.673760] gpt_parsed = {'lk': (1024, 2048), 'tee1': (3072, 10240), 'tee2': (13312, 10240), 'boot': (23552, 32768), 'recovery': (56320, 32768), 'logo': (89088, 7168), 'kb': (96256, 2048), 'dkb': (98304, 2048), 'MISC': (100352, 2048), 'vendor': (102400, 307200), 'system': (409600, 2252800), 'cache': (2662400, 1048576), 'userdata': (3710976, 11558879), '': (0, 1)}
[2021-02-18 00:14:41.674044] Check boot0
[2021-02-18 00:14:41.715671] Check rpmb
[2021-02-18 00:14:41.920020] Clear preloader header
[4 / 4]
[2021-02-18 00:14:42.016753] Downgrade rpmb
[2021-02-18 00:14:42.019082] Recheck rpmb
[2021-02-18 00:14:42.897696] rpmb downgrade ok
[2021-02-18 00:14:42.898870] Flash tz
[7265 / 7265]
[2021-02-18 00:16:24.096265] Flash lk
[613 / 613]
[2021-02-18 00:16:32.699113] Flash lk-payload
[6 / 6]
[2021-02-18 00:16:32.825264] Inject microloader
[2 / 2]
[2021-02-18 00:16:32.970991] Force fastboot
[2021-02-18 00:16:33.064034] Flash preloader
[294 / 294]
[2021-02-18 00:16:42.359225] Reboot

[root@sysresccd ~/Downloads/kamakiri/kamakiri-mantis-v1.2/kamakiri]# fastboot flash boot boot.img
Sending 'boot' (7528 KB) OKAY [ 0.352s]
Writing 'boot' OKAY [ 0.471s]
Finished. Total time: 0.834s

[root@sysresccd ~/Downloads/kamakiri/kamakiri-mantis-v1.2/kamakiri]# fastboot format userdata
mke2fs 1.44.4 (18-Aug-2018)
Creating filesystem with 1444859 4k blocks and 361440 inodes
Filesystem UUID: ba8f6462-0cbf-461d-bd70-9aa3d0894f01
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

Sending 'userdata' (2940 KB) OKAY [ 0.138s]
Writing 'userdata' OKAY [ 0.245s]
Finished. Total time: 0.495s
[root@sysresccd ~/Downloads/kamakiri/kamakiri-mantis-v1.2/kamakiri]# fastboot format cache
mke2fs 1.44.4 (18-Aug-2018)
Creating filesystem with 131072 4k blocks and 32768 inodes
Filesystem UUID: 4020bdca-c13f-4795-a325-fe9c853b9d6d
Superblock backups stored on blocks:
32768, 98304

Allocating group tables: done
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done

Sending 'cache' (324 KB) OKAY [ 0.017s]
Writing 'cache' OKAY [ 0.056s]
Finished. Total time: 0.099s

[root@sysresccd ~/Downloads/kamakiri/kamakiri-mantis-v1.2/kamakiri]# fastboot reboot
Rebooting OKAY [ 0.002s]
Finished. Total time: 0.052s
[root@sysresccd ~/Downloads/kamakiri/kamakiri-mantis-v1.2/kamakiri]#