Quick answer
Is 10 Mbps enough for IPTV?
It is enough for one HD stream, but 15 Mbps gives more headroom.
Speed by picture quality
SD needs about 5 Mbps. HD and Full HD need 10 to 15 Mbps. 4K needs 25 Mbps or more. These are per screen.
Multiple screens
Add the speeds together. Two HD screens need about 30 Mbps. Three screens with one in 4K need about 55 Mbps.
How to test properly
Run a speed test on the TV or Firestick itself, at the time you normally watch. A phone next to the router will always look faster than a stick behind the TV.