None of the FireTV devices have Gigabit Ethernet (1000Mbit).
They are limited to 100Mbit:
https://developer.amazon.com/docs/fire-t...?v=ftvgen2
Even if you added a known working USB3 > Gigabit Ethernet Adapter you will be limited to max. USB2 data transfer speeds, which is good enough for 4K HDR Bluray Rips playback.
Not all USB3 > Gigabit adapters work with all types of hardware. You have to be real careful you get a known working setup that someone has already tested.
All Smart TV's are 100Mbit only Ethernet as well because they are designed to only be low bitrate video streaming platforms. Not 4K Bluray Rips media players.
Get a media player with built in Gigabit Etherent if you want to reach near 1000Mbps data transfer speeds, and have it do it reliably.
MIMO AC WiFi (dual or more antennas) both on the Home router and Media player will be the best data transfer you will get Wirelessly.
The new 2018 FireTV Stick 4K has such MIMO WiFi capabilities.
