Exactly what I plan to do.
Search Youtube for FireTV Guru. He’ll get you fixed up and make updating easy.
I would offer, the Firestick is basically a modestly powered Android tablet without a screen. You have to be patient with it. In particular after you load Kodi and the plugins are updating. Best to just walk away and let it finish.
Streaming on Kodi is a little geeky. Behind the scenes, you are streaming bittorrents. Bittorrents aren’t all equal. There is a wide variety of quality. Again, be patient. If you get a bad stream, just stop and try another. You will soon learn which hosts work best.
The older and/or more obscure the movie is, the less likely you’ll find a good stream. Pretty much anything from 70’s forward you can find.
Good luck!