Ciao Lisa,
Scoppio del Carro in Florence is a major event and it can be pretty crowded, particularly Piazza del Duomo at the time the cart goes off. If you want to be there, the best viewing spots get taken up early so plan to get there early enough to stand for a while - I'd say around 9am. The best ones are the area between the cathedral and the baptistery, as the cart is placed in this area.
If you want to enjoy the parade in historical costume and see the cart closer up, I'd recommend heading and standing in the route the cart takes to get to Piazza del Duomo - anywhere between Piazza del Duomo and Piazza della Repubblica would be good. There will surely be people but I've personally found it is easier to manage the crowds along the streets than in the main piazza and I get to see more.... even if it means being further away from the cart itself as the piazza is really crowded by then. Once the cart arrives in Piazza Duomo, the cart gets set down and the fireworks get set up so it isn't as if the explosion takes place right away - so that gives you some time to get into the square and maneuver your way closer. This works best if you don't have kids, BTW ;-) Easier for 1 person to weave in closer and closer than more...