Although my Firyna has 2 locked rune slots, I do love using her as much as I can. For me, Firyna is one of the best Fire monsters.
She has a skill that gives immunity to burn and freeze, plus heavy special damage moves that heal all allies by 30% and adds 50% stamina to all allies. She also has a move that removes all negative effects from allies.
I enjoy using her in battle, she is brilliant when paired with damage dealers or monsters with high stamina usage (Shallinar, Igursus).