I think that it might be a bit weak unless you're fighting a lot of fire elemental creatures. In general, i don't like a lot of the enemy specific items because, for the most part, they're either useless[if you rarely come against fire elementals], or its a way to get a stronger ability than you would normally[if you come up a lot of fire elementals].
It would probably be better balance wise if it applied to any creature who was taking ongoing fire damage or was resistant to fire[naturally, not by item]. This way, players can trigger the ability by using an ongoing fire power first, or they can use it on fire elemental based creatures.
I think these kinds of restrictions are better for balance than type based ones since they only allow you the stronger power if you meet a condition that requires you to spend actions that may or may not be optimal.