I'd say Spottedleaf. Spottedleaf was his first love, and in the warriors 猫 guide, it 说 that if Spottedleaf was younger, and a warrior apprentice, then maybe they could have been together. Spottedleaf also, later in Starlight, Spottedleaf 说 that she would use all of Starclan's power to help Firestar. I don't think it would ever be Cinderpelt because Firestar was Cinderpelt's first mentor, but Sandstorm would also be a reasonable coice.
Cinderpelt!!! She always loved Firestar and it's not her fault she was a medicine cat! I'm glad she got a 秒 life! Spottedleaf was just Firestars childhood crush. Sandstorm is the worst mate ever! When Firestar was new to the Clan, she hated him. Then he becomes 流行的 and she's all over him! CinderXFire for ever!!