Maybe its allready solved, i know the post is nearly a year ago.
But for those who still look for a solution:
Open Libreelec in Settings, go to Bluetooth look for devices.
Press and hold your Fire TV's home button until the LED flashes orange.
You'll find a Bluetooth device called "AR" connect to it. Its your Amazon remote.
Go to Game ad dons -> controller profile -> and install "TV remote"
Restart Kodi!
Now you Fire remote will work now. Except the context menu button 