Here is a nice list of all the best FREE Legal apps that can easily be side loaded to a Fire TV. Many of these you will already find in the Amazon store so side loading is not even needed.

Should you want to add other TV streaming apps, the easiest method is to get the FileLinked app and then add the E×TREME FILE×LINKED 2019 Green Monster app store. (You Will Need to first Join their MeWe group for the code)
And once you are on MeWe be sure to also join our Cord Cutters Group.
FileLinked will automatically download the apps of your choice from a long list of really cool apps directly to your Fire TV. And this is about the easiest way to add a lot of great apps that are not in the Android or Amazon store without any fuss.

Another group to join on MeWe is "Fishbone Rocks" |Android TV, iptv, m3u, apk's, sports, tv box reviews, group giveaways, free live tv, movies, music, free ebooks and more.