A quick search for Capodanno in Firenze gave me this.

http://www.capodannofirenze.org/

The weather is something you might consider if you would like to spend the eve outside in a piazza attending concerts and festivities.

Thanks Milena those are some cool spots for Florence!

WesMan,

You can do a "capodanno" search for every city you mentioned and see what's happening. You can even do that kind of a search on YouTube and come up with videos from last year for each city.

You can also do a search for "previsioni meteo" for all the same cities and see a long term weather forecast. Not very reliable when it comes to rain, but often in the ballpark when it comes to temperatures.

Nobody knows what you want to do on NYE. Italians usually go home and play cards with their families. Young adults go hang out somewhere like a favorite bar or head to a big piazza to look at 10 minutes of fireworks. Rome is rowdy. Venice tries to commercialize it but the town is otherwise dead. In Milan and Florence, young people basically look to hook up with their friends. Napoli is wild, and they've been known to set half the town on fire with explosions.

I spent last NYE in Rome and had a blast! There were tons of people on the streets, many closed to vehicle traffic for the night.
Several of the Piazza's had fireworks and outdoor parties going on. There was a concert in front of the Forum.
Grab a bottle of Champagne from a street vendor, and an umbrella (you'll find out why ) and just follow the crowd to celebrate the count down at midnight!