Asking for TWRP test 8 or 9 is like asking for a beta. TWRP v7 which is in the OP (First Post) is the Final version of all those "test". So that is what you need.
This is the best guide out there:
http://www.aftvnews.com/how-to-install-twrp-custom-recovery-on-a-rooted-fire-tv-1/
Make sure you do the Full boot loader Unlocking if possible too. Go thru all the prerequisite requirements one by one. Just replace the part about v6 to v7. In your stated case above you will only follow
"Section 1: Fire TV 1 on 51.1.6.3 or older" & the TWRP file you will use is the TWRP image
"July 4, 2016 - TWRP Image 3.0.0-7 (md5sum: cda1a1c1cdd3e25a1caa86f5a5de1eee)". Once TWRP is installed correctly. That means you can reboot to it using a USB keyboard. You are set. All future updates of TWRP will happen thru the PreRooted ROM (Unless stated otherwise by RBox).
Make sure you use MD5 files for hash checking anything you are flashing thru TWRP. It really helps with file integrity. You don't need to do a Factory Reset & NEVER DO A /SYSTEM wipe unless you are an expert. When ever you do a /SYSTEM wipe you have to flash a PreRooted ROM before rebooting or you will brick your FTV1. Flashing the PreRooted ROM will wipe /System for you. So you never have to go out of your way to do that. You can always do a Cache & Dalvik wipe of you are having any issues. Those can help. But are rarely needed. I myself keep a USB Stick with all the files I need just for updating my FTV1's. Makes things a lot simpler.
If you have ?'s or don't understand something ask NOW. A lot of folks that brick their FTV1's wait to ask for help in the middle of the process or after it's to late.