Hello Sara!
I'm not an expert, but I think I might help you. Firstly, I would not try to translate fireheart in one word. I'm a spanish speaker, and some words work different in other languages. For example, there's no word for fireheart in spanish, we say instead corazón de fuego, literally in english: heart of fire. I think you should translate heart of fire, instead fireheart.
I hope I've helped you.