It will tell you it is not compatible, just ignore that. Buy the app, sync amazon content to your firetv and manually launch Kodi once. After that it will just be in your Recent listing. It will not appear in your apps listing though.

This is the app I am referring to.

There's no need to use a sacrifice app for Kodi specifically anymore, it's available on their appstore, and even though it isn't compatible with the firetv "purchasing" it and syncing content will allow it to show up in the recent section of the launcher, though it'll still have to be sideloaded and updated the way it always has Kodi on Amazon