Now it's 2024. I don't have a Fire Phone, but the Fire Phone runs Android 4. I have a few old Android 4 tablets, and I don't bother using them. You can't even run a modern version of a web browser on Android 4! As Nguyen1 said: Apps are likely to be old.
I don't think there's a custom ROM for this phone, so it's stuck on Android 4. However, the phone is 4G (that's culler technology, nothing to do with the Android version number), so it will probably still be able to connect to cellular networks and make phone calls.