Peru midfielder Juan Vargas gave Fiorentina a halftime lead with an angled shot and Rossi scored what turned out to be the decisive goal with a 54th-minute penalty awarded for handball by Bosko Jankovic, who was sent off.
It was Rossi's 12th goal of a season which has seen him make an impressive comeback after a knee injury which sidelined him for the best part of two years.
Despite playing with a man less, Verona pulled one back in the 72nd minute when Jorginho scored from a rebound.
Fiorentina are equal fourth with Inter Milan on 27 points while promoted Verona are sixth with 22.
Napoli were away to Lazio in the later game (2000 GMT).
(Reporting by Brian Homewood; Editing by Rex Gowar)
