Insigne produced a moment of magic on the 38th minute, winning the ball inside his own half and embarking on a winding dribble past several opposition defenders, before playing an inch-perfect pass behind the defence for Hirving Lozano to tap in.
The Napoli captain's piece of breath-taking individual skill at the Stadio Diego Armando Maradona drew comparisons with the legendary Argentinian forward, but the hosts were not done as Piotr Zielinski arrowed in a fourth before halftime.
Gennaro Gattuso's side were clinical and the scoreline left struggling Fiorentina stunned, as Napoli had six shots on goal to the visitors' five in the first half but found the net with each of their four efforts on target.
Insigne added his second in the 71st minute by converting a penalty after Tiemoue Bakayoko was tripped by Gaetano Castrovilli in the box, and substitute Matteo Politano completed the rout late on with a well-placed finish.
Napoli climbed above AS Roma on goal difference into third place on 34 points. Fiorentina are 14th with 18 points.