It appears that the method to root the Fire TV 2 using an A-to-A USB cable, discovered by Eric (a.k.a zeroepoch) and later improved by rbox, no longer works with software version 184.108.40.206. When connecting the Fire TV 2 via USB, the preloader handshake works, but the script is unable to read the device’s partitions. I don’t know if this is something that can be resolved by updating the rooting script, but for now, the last rootable software version for the Fire TV 2 is version 220.127.116.11 using this guide. Fire TV 2’s running 5.0.0 thru 5.0.5 should still use the simpler KingRoot method to root.
There may still be hope for rooting the Fire TV 2 running software version 18.104.22.168. It appears the Fire TV Stick can be rooted with 22.214.171.124 using an app called Kingo Root. I have tried running Kingo Root on a Fire Tv 2 running 126.96.36.199 but it did not root the device. If more can be learned about the Kingo Root process, and why it sometimes works on Fire TV Sticks running software version 188.8.131.52, perhaps it can be adapted to the Fire TV 1 and 2.
The good news is it seems Fire OS 5 is more likely than Fire OS 3 to update incrementally through the various software updates. I allowed a Fire TV 2 on 5.0.4 to update and it updated to 5.0.5, then to 184.108.40.206, and then to 220.127.116.11. In the past, Fire TVs were more likely to jump directly to the latest software version, so if you buy a new Fire TV and are unable to prevent an update from installing, there’s a chance it will update to a rootable version and not straight to 18.104.22.168.