I lived in florence for a year and got a super cheap beater bike to go two miles to school every day. With the exception of a motorino, I don't think there could have been a faster way to get there.
Watch out for cars and motorinos clogging the streets and if there are no cars or bikes, watch out for unorganized crowds. Some sidewalks will work but some of them are about a foot wide. The safest way I found to travel by bike is to try to stick close to other bikers. Watch out for the touristy areas because tourists there travel in huuuge packs that you will need a machete to cut through.
I remember how rampant bike theft was when I was there- It seemed like every bike on the road had been stolen and resold and stolen and resold at least a couple times in its life. Be careful if your foldy is nice
The underside-of-the-neck-backhand-flickoff is one of the rudest gestures you can do there, and "vafanculo" or "stronzo" are pretty strong words. Watch who you flip off, if the typical italian guy sees you insult his mama you are in serious trouble!
As a last resort, try a strong headbutt to the sternum... hehehe