Thanks @sd_shadow for the reply.
In recovery mode, TWRP reports that it is version 18.104.22.168
I did try the "fastboot devices" command. When I am in the initial FFF screen (blue & white kindle fire logo with "Press power button for boot menu") or I am in the FFF menu with the 3 boot choices at the bottom (Normal Boot, Recovery, Reset Boot Mode), the "fastboot devices" command returns "0123456789ABCDEF fastboot". I didn't make up that number. It reports the hexadecimal digits in order from 0 through F, followed by the word "fastboot". It is pretty apparent that this is not a valid device ID number, but it is reporting something. I just don't know what it means.
If I am in any screen other than those, the "fastboot devices" command returns nothing, and it goes back at the command prompt.
the "0123456789ABCDEF fastboot" could be good, I believe that is what i usually get, and would go ahead and flash twrp.
fastboot flash recovery openrecovery-twrp-22.214.171.124-otter.img (or whatever version you are trying)
or try the reboot command