No. Firestar even told Sandstorm in Firestar's Quest, right in front of Spottedleaf, that she would never be second best to him and he would love her for all the moons to come. And, after Spottedleaf left, he again affirmed to Sandstorm: "There'll never be another cat in the forest like her. But she's not you, Sandstorm."
Spottedleaf herself also confirmed Firestar's love for Sandstorm, and said: "Firestar and I never discovered what we might have meant to each other." She had only been alive for a short time after he arrived to the forest, and she was destined to be a medicine cat. It would have been a similar situation to Yellowfang/Raggedstar and Leafpool/Crowfeather, and none of those pairings have ended up together so far. Their love was more of a what if, while his love for Sandstorm is a what is.