AC Milan rose to second after substitute Klaas Jan Huntelaar netted his first two goals for the club deep into injury time to seal a 2-0 win at Catania.
The Dutch striker has floundered since a close season move from Real Madrid but he finally showed his worth by powering in Filippo Inzaghi's layoff and then producing a perfect chip.
Argentina striker Milito converted Inter's spot-kick after being brought down, having earlier missed an easy chance.
Champions Inter, chasing their fifth straight scudetto, thought they had gone ahead early in the second half when Walter Samuel's header was controversially ruled out for shirt tugging.
Alberto Gilardino hit the post in Fiorentina's only real chance and Inter forward Samuel Eto'o's shaky form continued when he wasted a late one-on-one with goalkeeper Sebastien Frey.
"I have just one duty - to be honest. I don't need to win anything, not the championship, not the Champions League," Inter coach Jose Mourinho told Sky television when asked if his side must now focus more on Europe with Serie A becoming a stroll.
"My team won and I am good again," he joked, referring to criticism after the midweek Champions League loss at Barcelona.
Promoted Parma moved up to joint fourth with Sampdoria, beaten 3-0 by sixth-placed Genoa in Saturday's derby, thanks to a 1-1 home draw with Napoli.
Lazio's poor season drags on after a 0-0 draw at home to Bologna with their former coach Delio Rossi losing in his first league game in charge of Palermo, beaten 1-0 at Chievo.
