The 28-year-old, who joined from Barcelona in 2021, "moves on to a new challenge" following the expiry of his contract.
Real Betis' sporting director Manu Fajardo said last week Firpo wants to return to the Spanish club, but that a deal is not yet finalised.
Firpo started his professional career with Betis before playing for Barcelona and Leeds.
The Dominican Republic international spent two seasons with Leeds in the Premier League before helping the club win promotion from the Championship as league winners last season.
Firpo made 35 appearances in all competitions for Leeds in his final season, providing 10 assists and scoring four goals.
