[Fire General] NCode!:
At first it tried to follow this guide (especially the video linked from there) to flash, but with the update to 5.1.2 much of the bootloader functionality needed for rooting/flashing seems to be locked by amazon so that the root process of the SuperTool (Version from March 12th 2016, the MD5 of the AmazonFire5thGenSuperTool.zip was 8d813f3eacef9e932af5c366769b505e, not the one given on the download page where the file came from) failed.

So following the recommendation from here I tried to use KingRoot for rooting.

Rooting

I downloaded and installed KingRoot 4.8.2 from here. (file name "NewKingrootV4.82_C138_B250_xda_release_2016_03_09_105243.apk", md5 11d61bfa1ab79ad3bb55cbaa1ebd6fc7).

Running

adb install NewKingrootV4.82_C138_B250_xda_release_2016_03_09_105243.apk

worked flawlessly.

Now I started KingRoot on my Kindle. KingRoot said that root is (currently) not available on this device, but that it can try rooting.

I connected to WiFi and let KingRoot download its instructions. At probably about 20% into the rooting process, the device rebooted.

Afterwards I had to restart KingRoot and after some further thinking and reaching ~27% it said that the rooting process failed.

But I didn't give up, rebooted the device, restarted the KingRoot rooting process and voilà - this time it ran through without rebooting and I got root.

Installing SuperSu and FlashFire

Now on getting SuperSu to work for using FlashFire, which won't work with KingRoot.

I extracted the Superuser.apk from the official .zip distribution obtained from here ("UPDATE-SuperSU-v2.65-20151226141550.zip"), but you can probably also get it from the playstore if you got that installed. The md5 of the Superuser.apk was b4a76bb94927431f0d5b617836fb3f9c.

After installing SuperSu via adb install it complained that another su app was installed and its binaries needed an updated. Neither the automated removal of KingRoot nor the update of the binaries worked.

So following this guide I downloaded the mrw.zip that was attached to the third post (md5 1c769fe61e1fe5952b81fb80965c4d6b) and then unzipped and pushed the contained "mrw" folder to /sdcard/ on my kindle using

adb push mrw /sdcard/mrw

As the script reported some failures (which is	supposedly normal) and the SuperSu didn't come any further, I checked the mrw/root.sh script.

After replacing ´cat sdcard/mrw/busybox > /system/bin/busybox´ with ´cat /sdcard/mrw/busybox > /system/bin/busybox´ in line 17 (see the missing root slash?) I got the script to work (at least when not being started from the root dir).

The important part from the skript (see below) now ran without errors.

The two

>/dev/null 2>&1

files, /system/xbin/kugote and /system/xbin/pidof did realy not exist, so that didn't matter.

Thanks beingtejas for the skript (again, see the mrw.zip from the link above for the full, working version of the script):

cat /sdcard/mrw/busybox > /system/bin/busybox #this was line 17 with the missing slash
chown 0.1000 /system/bin/busybox
chmod 0755 /system/bin/busybox
busybox chattr -ia /system/xbin/ku.sud
rm /system/xbin/ku.sud
busybox chattr -ia /system/xbin/kugote >/dev/null 2>&1
rm /system/xbin/kugote >/dev/null 2>&1
busybox chattr -ia /system/xbin/su
rm /system/xbin/su
busybox chattr -ia /system/xbin/supolicy
rm /system/xbin/supolicy
busybox chattr -ia /system/xbin/pidof >/dev/null 2>&1
rm /system/xbin/pidof >/dev/null 2>&1
cat /sdcard/mrw/su > /system/xbin/su
cat /sdcard/mrw/su > /system/xbin/daemonsu
cat /sdcard/mrw/su > /system/xbin/sugote
cat /system/bin/sh > /system/xbin/sugote-mksh
chown 0.0 /system/xbin/su
chmod 6755 /system/xbin/su
chown 0.0 /system/xbin/sugote
chmod 0755 /system/xbin/sugote
chown 0.0 /system/xbin/sugote-mksh
chmod 0755 /system/xbin/sugote-mksh
chown 0.0 /system/xbin/daemonsu
chmod 0755 /system/xbin/daemonsu
daemonsu -d

Now KingRoot was uninstalled, the SuperSu update ran through and I rebooted the device as recommended.

Flashing

Afterwards the FlashFire installation ran fine with the SuperTool as described in the beginning and I could continue the normal installation.

After a very long AOSP boot sequence I now got the Fire Nexus ROM running nicely ob my kindle.

I mainly wrote this as documentation for myself but wanted to share it for other users facing the same problems with the updated bootloader.

If this post actually belongs elsewhere, please tell me to move it, as it is my first one and I couldn't find a better place to put it.