Pioli said he felt he had been forced to leave due to a lack of faith from the club's management
"I have always assumed my responsibilities; I have always ensured my professionalism, respect and maximum commitment," Pioli told ANSA.
"I reluctantly see myself forced to leave because my professional and, more importantly, human qualities have been questioned."
Fiortentina have moved quickly to replace Pioli, bringing in Montella on a deal until June 2021.
Fiorentina, who have won only one match in all competitions since a 7-1 Coppa Italia thrashing of Roma on January 30, sit 10th in the Serie A table with seven games to go, starting with the visit of Bologna on Sunday.