[Kindle Fire HDX 7" & 8.9" Q&A, Help & Troubleshoot] velcroghost:
Okay so I figured out bulk mode method on linux for myself, so I will share it here for anyone who needs instructions:

1. Make sure that dd, fdisk, adb, and fastboot are installed and available on your system. In Debian based distributions (including Ubuntu, Linux Mint, Debian itself, etc.), you can use apt to download them if they aren't already installed. Additionally, make sure your HDX is charged above 80%. Do this before you continue with the rest of the steps.

2. Download the unlock.zip and appropriate aboot zip file from the attachments of this thread and extract both of them in a folder in your home directory. Additionally, download the version of TWRP appropriate for your device from this other thread and place it into the same folder.

For the HDX 7" you will want the thor files, and for HDX 8.9" you will want the apollo files. You should have a file beginning with "emmc" and ending in ".mbn", as well as a file beginning with "[thor/apollo]-twrp" an ending with ".img"

3. Turn on ADB Debugging Mode in the HDX by tapping the build number in settings 7 times. Connect the HDX to your computer via USB.

4. In a terminal, navigate to the folder where you placed the previously mentioned files and execute:

adb devices

This command should show your device. If it says unauthorized, you may need to authorize debugging mode on your HDX for this computer. It should pop up a message asking you to confirm. If it does not list your device, your Android toolset may not be installed correctly.

5. Next type:

chmod +x get_code.sh
chmod +x unlock.sh
./get_code.sh

This should mark the .sh scripts as executable and generate an unlock.code file within the same folder. Keep it here for later.

6. If your device shows up after typing in the command in step 4, you may now type:

adb reboot bootloader

This will take you into fastboot mode. The HDX should reboot into a screen that says [Fastboot] at the top. The remaining steps are where accuracy and care is critical in order to avoid bricking your device, so if you have doubts proceed with caution.

7. You now need to put the device into bulk mode so that you can overwrite the aboot partition with a version that is vulnerable, as well as the default recovery with TWRP. Type the following commands to proceed:

fastboot -i 0x1949 erase aboot
fastboot -i 0x1949 reboot

The HDX should restart and show up with a black screen. This is normal. Your Linux desktop should start to see some partitions show up. Dismiss any prompts to open these partitions in your file browser.

8. Next, run fdisk:

sudo fdisk -l

You should see a new block device/drive show up other than your hard drive. It should have well over 16 partitions. Partition 6 should have 4096 sectors and partition 18 should have 20480. These are your aboot and recovery partitions. Take note of the three letter code of the HDX's internal storage within fdisk. This should look like "sd[x]" where x is a letter.

9. Now you will need to overwrite the aboot and recovery partitions using the utility called dd. This will flash an image to the partition chosen. Make sure you are doing this from the same folder that contains the files you downloaded before. The vulnerable aboot should start with "emmc" and end with the extension ".mbn", and the recovery should start with "[apollo/thor]-twrp" and end with the extension ".img".

Run the following commands, substituting [X] for the three letter code representing the HDX in fdisk, [Y] for the vulnerable aboot .mbn that you downloaded, and [Z] for the TWRP recovery image you downloaded:

sudo dd of=/dev/[X]6 if=[Y] conv=notrunc bs=64k oflag=direct

sudo dd of=/dev/[X]18 if=[Z] conv=notrunc bs=64k oflag=direct

For example, if the HDX is assigned to sdb it should look something like this (but make sure you're using the files you downloaded specifically - use tab to autocomplete).:

sudo dd of=/dev/sdb6 if=emmc_appsboot-13.3.2.3.2_user_323001720.mbn conv=notrunc bs=64k oflag=direct

sudo dd of=/dev/sdb18 if=thor_twrp-3.2.3-1.img conv=notrunc bs=64k oflag=direct

10. If the overwrites with dd work successfully, wait a few minutes and then hold the power button until it hopefully reboots back into fastboot mode. Now run:

./unlock.sh

This should unlock your bootloader.

11. Once all of the above steps are done, you should be able to hold down power + volume up to boot into TWRP recovery mode. From here you can flash a custom ROM like LineageOS. You can find versions of custom ROMs compatible with your HDX in this subforum.