I have a Fire HD8 7th Gen (5.7.0.0) and had a similar problem last night, but was able to get everything working. Specifically, the problem I had was that Windows automatically installed drivers for the Fire but refused to install the Google drivers, even when I tried it manually. In the past I have used ADB with this Fire, as well as, some older android phones, so I think that was part of the issue.
After downloading and extracting the Google drivers to my PC, and having FireToolbox running (but not happy 'cause drivers are missing) here's what I did:
1. With Kindle connected to PC and USB debugging enabled, go to admin account on PC and open Device Manager.
2. In the Device Manager, check to see if you have "Fire" under "Portable Devices" in addition to something else. For me, the something else is "Android Phone". (Maybe a Roku shows up the same way?)
3. Under "Portable Devices", I uninstalled the "Fire" (right-click, uninstall)
4. Then, still in the Device Manager, I went and modified the "Android Phone" basically following Method #2
https://forum.xda-developers.com/t/windows-tool-fire-toolbox-v30-0.3889604/post-82681973
For me, that was enough to get FireToolbox to work with my Fire, block the annoying ads, etc and install Nova. Windows has already done it's automatic thing and put "Fire" back under "Portable Devices" in the Device Manager with the default MTP drivers, but it hasn't stopped FireToolbox from working. I got the idea of deleting the non-ADB driver from reading
https://developer.amazon.com/docs/fire-tablets/connecting-adb-to-device.html#troubleshooting I'm sure there are some much smarter people on this forum who can figure out what's going on, but hopefully this helps as a work-around.
Thank you Datastream33 for this awesome tool, I'll be buying you a coffee