Do not hire a car for whilst you are in Florence. Most of the city centre is not accessible to cars, and it will turn to be just a hindrance.
As for good prices, try www.nova.co.uk. They give very good prices. We have used them in the UK, and have recommended them for Greece, and everybody has been very satisfied.
Indeed you won't need a car inside either of those cities; in fact it would be advisable not to get too close to naples at all in a rental car, both cuz it might get stolen or broke into and because you will risk your sanity trying to drive there. Even Florence is not great driving for those not used to really crazy traffic. If you need to get between those cities directly, take the eurostar train (4.5 hours). If you plan on seeing things in between, ok go for it. Pick up the car when you're ready to leave florence. Leave it as far away from central naples as you can and take public transport from there!!
Most people find autoeurope to be the best priced car rental. Make sure you get the full insurance called 'casco completo'. It doesn't cover broken windows though which might be the norm in naples.
Thank you all for your advice. I ended up hiring with europcar on our last day in Florence and therefore avoided driving in the city centre for any length of time. They allowed us to drop the car off in Sorrento for no extra cost. The car did allow us to explore Tuscany, being based deep in the countryside outside San Gimignano