Ideas and suggestions.

It would be better to give firebirds immunity to fire again.

Why? If people complain about Flax, it's more about its overpoweredness than its weakness. Removing the immunity would significantly increase the difficulty – scrolls with Arcomage became much more accessible because they no longer require GM4 to use. And firebirds can appear for any faction, like the Crimson Towers. So, removing the immunity is a very important thing. If someone wants immunity, let them upgrade the firebirds.