[Kindle Fire HDX 7" & 8.9" General] ONYXis:
Hello. At first, I did not invent anything new, just checked some my guess on a other motherboard. All thanks and credits to our great developers. As always, all at your own risk.

It does not work on the Fire HDX 8.9 (Saturn)!

All steps in this manual are not necessary but they are present for maximum safety. So I highly recommend do anything exactly in this way. Sorry my English as always =)

Update2 - actual method is https://xdaforums.com/showpost.php?p=75284993&postcount=1006

Update: now you can use updated draxie's utility - http://xdaforums.com/kindle-fire-hdx/general/multi-platform-1-click-bootloader-t3241014

Prerequisites for Installation

- Root

- Installed adb and fastboot drivers - official - https://drive.google.com/open?id=0B2twXJIOgv-UWWdwRl9TQS11b0k (if your system language not English, after fail navigate to "Program Files (x86)\Lab126\drivers" and run dpinst.exe /EL or switch to English =) for x64 need to disable driver signature verification before install ) also you can use pdanet drivers - http://xdaforums.com/showpost.php?p=59268023&postcount=8

Manual:

1. Create unlock file following this instruction - https://xdaforums.com/kindle-fire-h...r-firmware-t3463982/post70881555#post70881555

2. Flash old vulnarable aboot and cubed twrp (just in case). Check that all these commands executed without errors. If you'll get one - read second post below. If your firmware <=13(14)3.2.3.2 skip this step.

Download aboot and twrp for Thor (Kindle Fire HDX 7) https://drive.google.com/open?id=0B2twXJIOgv-UMGxXMUZPZTlZTUk or for Apollo (Kindle Fire HDX 8.9) - https://drive.google.com/open?id=0B2twXJIOgv-URzJDQkczNzRLaHM - and put this two files (twrp_cubed.img and aboot_vuln.mbn) into root of your kindle internal storage.

Run:

adb shell
su
dd if=/sdcard/twrp_cubed.img of=/dev/block/platform/msm_sdcc.1/by-name/recovery
dd if=/sdcard/aboot_vuln.mbn of=/dev/block/platform/msm_sdcc.1/by-name/aboot

Now you have working twrp recovery. It already works even without unlocked bootloader. You could boot into it by holding volUP during grey kindle logo. But no need to flash anything until unlock. At this point this is just emergency tool if something goes wrong =)

3. Flash unlock file.

Now, if you reboot, you will go straight into fastboot because of old aboot - newest boot.img can't load with it. If your firmware <13(14).4.1.1 you need run "adb reboot bootloader" to boot into fastboot.

Time to flash your unlock file.

fastboot -i 0x1949 flash unlock 0xmmssssssss.unlock

You must obtain "unlock code is correct".

Grats. You are perfect =)

You can flash:

CM13 - http://xdaforums.com/kindle-fire-hdx/orig-development/rom-cm-13-kindle-hdx-2015-11-29-t3259732

CM 12.1 - http://xdaforums.com/kindle-fire-hdx/development/rom-cm-12-unofficial-apollo-thor-t3050199

Or stock repacked latest 4.5.5.2 rom - https://drive.google.com/open?id=0B2twXJIOgv-UVFFtN2RYNXNUZ0k (13.x - thor, 14.x - apollo)

Do not flash original stock firmwares.

Regards and thank to all - [MENTION=438903]dpeddi[/MENTION], [MENTION=4450207]vortox[/MENTION], [MENTION=5828306]draxie[/MENTION], [MENTION=5749760]ggow[/MENTION], [MENTION=2918486]Ralekdev[/MENTION], [MENTION=2376614]jcase[/MENTION], [MENTION=4243514]Hashcode[/MENTION]

And greatest thanks for motherboard for my experiments to [MENTION=3039035]MahmudS[/MENTION] !