Carlo Ancelotti's Napoli had fallen to their first defeat of the season before the international break 3-0 to Sampdoria in Genoa.
But Insigne broke through after 79 minutes in the San Paolo Stadium, whipping in his 50th Serie A goal following a cross from substitute Arkadiusz Milik.
It handed Fiorentina -- who ended Napoli's title hopes last April -- their first defeat of the season.
"In Genoa we weren't ill, we just had a little cold," said Ancelotti.
"Today we were more balanced and effective. We played the game we wanted today even if it wasn't an exceptional level.
"It's pleasant, exciting and wonderful to be back in the Champions League. We've got a tough group and will try to get a good start."
Napoli move up to second with nine points, equal with champions Juventus who host Sassuolo on Sunday with Cristiano Ronaldo still waiting for his first goal.
Inter Milan's on-loan defender Federico Dimarco sealed victory for his new club Parma after 79 minutes in the San Siro.
"It's a painful defeat," said Inter coach Luciano Spalletti whose side drop to 12th just ahead of Parma.
Inter have only four points from as many games ahead of their return to the Champions League for the first time since the 2011-2012 season.
"When you find yourself with four points from four games in front of this crowd and considering the fixture list, it becomes difficult to find excuses," added Spalletti.
"We need to do more."
The 20-year-old Dimarco came on as a substitute just after the interval and unleashed a fierce long-distance strike into the top corner to score his first Serie A goal.
It gave promoted Parma their first league win this season, and first victory at the San Siro since May 1999.
The two-time UEFA Cup winners were declared bankrupt and demoted to the amateur leagues in 2015, before back-to-back promotions saw them return to Serie A.
"We were brave and I think fortune favours the brave," said Parma coach Roberto D'Aversa.
Spalletti rang the changes with a view to Tuesday's Champions League opener, resting last year's top scorer Mauro Icardi, who had been struggling with a muscular problem with right-back Sime Vrsaljko out with a knee injury.
But the hosts failed to consistently threaten despite early chances for new signing Radja Nainggolan and Marcelo Brozovic, with Ivan Perisic also forcing a save from Parma goalkeeper Luigi Sepe.
Icardi came on after the break for Keita Balde and was also denied by Sepe.
But Parma opened the scoring in style when full-back Dimarco dispossessed Brozovic in midfield and fired past Inter goalkeeper Samir Handanovic.
Former Arsenal and Roma winger Gervinho nearly added a second but saw his shot saved by Handanovic.
Inter had a penalty appeal for a Dimarco handball turned down after consultation with VAR.
"I'm sorry to have scored at Inter because I'm an Inter fan, but I know that this is a very important goal for us," said Dimarco, an Inter youth academy graduate.
Meanwhile, French forward Gregoire Defrel scored his second brace in as many games as Sampdoria continued their form by hammering promoted Frosinone 5-0.
Defrel scored after 54 and 86 minutes to go top of the scorers standings with four goals.
Fabio Quagliarella had opened after ten minutes at Stadio Benito Stirpe with Gianluca Caprari getting the second just after the break. Dawid Kownacki converted a penalty seven minutes from time as Sampdoria move up to fourth.