I got a Fire HD8, 6th Gen. around Christmas. It was on 5.3.1.0. I haven't turned on WiFi, because I think 5.3.1.0 may be a good version for rooting. The Fire 7" 5th gen. is supposed to be able to be rooted if the device is on 5.3.1.0.
I would like to know if the HD8 6th gen and 7" 5th gen. are identical for rooting/ downgrading etc. if they are running the exact same firmware.
When I read the YouTube comments for the several of Rootjunky's rooting vids for the 7" 5th gen., I see so may rooting failures and brickings that I am very hesitant to try anything. I can't decide whether the high rate of failures in those YT comments are due to many of the would be rooters not knowing much about what they're doing or whether Rootjunky's "supertool" is flawed and dangerous.
Rootjunky's supertool works for 7" and only 7" for rooting (some of the other things sometimes work of other devices). For example, I rooted a 7" with 5.3.1 this Dec using his tool (however, I think think the newer firmware is no longer rootable)
The 2015 HD8 is rootable on some firmware versions.
As of this Saturday, using a different method than rootjunky, you can root the 2016 HG8"/10" on 5.3.1 and 5.3.1.1.
If you know what you are doing, little risk. If you don't you can brick. Most bricks happen by downgrading too far.
For me, rooting 7" was needed to run Link2SD which you need since it only has 8GB.
The 8" had 16GB so less of an issue but I hated the firelauncher and the work arounds were clunky. Both are now rooted. One with custom ROM and one with OTA disabled and nova launcher, titanium backup and Link2SD all running with root access.