Napoli continued to lose ground on the league leaders, after losing 3-1 to Palermo on Saturday night.
The home side raced into a two-goal lead inside the opening 35 minutes thanks to goals from Franco Vázquez and Achraf Lazaar.
Luca Rigoni sealed the win for Palermo in the 66th minute, before Manolo Gabbiadini grabbed a consolation for Napoli in the 83rd minute.
Fiorentina's hopes finishing in the Champions League spot was further enhanced following their 3-1 win over Sassulo on Saturday evening.
On-loan Chelsea winger, Mohammed Salah, bagged his first goal for Fiorentina in the 30th minute, before Khouma Babacar doubked the lead two minutes later.
Babacar grabbed his second and his side's third in the 62nd minute, while Domenico Berardi got a consolation strike in the 67th minute.