Flumefire Rage is the feat that adds +1 damage for every die you roll when casting a fire damage spell, at the risk of becoming Fatigued.
If you are raging, the risk of getting Fatigued lessens, and the damage bonus increases; on the other hand, usually, you need things like Furious Spell if you want to cast under such circumstances.
What intrigues me, though, is the following passage in the Flumfire Rage description:
This feat does not otherwise grant you the ability to cast spells while raging.
Does it mean that while this feat doesn’t help you cast any non-blasting spells when rating, you don’t need Furious Spell when casting a Fireball?