Remember that restaurants have separate prices for food to go or eaten standing up versus sit down service; don't try to sit at a table after paying for food or coffee from the restaurant's to go booth. Also ask always beforehand for the price if you want to sit at a table. Otherwise you might be uncomfortably surprised. Cappuccino al banco i.e. standing up might cost ?1-3; but at a table ?4.
Florence's food can be as much of a treat to the palate as the art is a treat to the eye! There is good food for any price range, from fine restaurants to take out food from window stands. The best price/quality ratio you will find outside the historical center where normal Italians go to eat. The worst ratio is probably in the neighbourhood of Mercato di San Lorenzo where there are a lot of tourist restaurants, while many of the best restaurants in the city are found in the Santa Croce district. In some, requests for pizza may be met with a rebuff. For local pizza look for small shops near the Duomo.
The best lunch places don't always turn out to be the best dinner places. Dinner in Florence really starts sometime between 19:00 and 21:00. If a place looks like they're preparing to close before 20:00, it might not be the best option for dinner. Reheated pasta is not very tasty.
Typical Tuscan courses include Bistecca alla fiorentina which is huge t-bone steak weighing from 500 to 1500 grams. It has always price given per 100 grams e.g. 3,5? etto (etto is "hecto" pronounced in Italian). Crostini toscani are crostini with tuscan liver paté.
There is also a uniqely Florentine fast food with a 1000-year history - lampredotto, a kind of tripe (cow stomach, or calf for preference, but a different part than the more familiar white "honeycomb" kind, dark brown in color; the name comes from its wrinkled appearance, which apparently reminds locals of a lamprey fish). The trippaio set their carts in the public squares in the center, dishing out the delicacy straight from the cauldron in which it is being boiled with herbs and tomatoes, chopping it and slapping the portions between halves of a Tuscan roll; the top is dipped in the broth. A mild green parsley- or basil-based sauce or a hot red one goes with it.
There are many gelato (Italian ice cream) stands; some connoisseurs consider the better Florentine gelato the finest in the world. Often gelato is made in the bar where you buy it. Because of this there are many exotic flavors of ice cream like watermelon, spumante or garlic. It's hard to find a gelato place open very late, so after dinner might not be an option. Near the Duomo though, there are a few places open after 22:00.
Tuscany is also the wellspring of cantuccini, also called biscotti di Prato. (Please note that in Italian, the singular of biscotti is un biscotto.) It's traditional to enjoy them after a meal by dipping them in Vin Santo ("Holy Wine"), a concentrated wine made from late-harvested grapes, but you can also buy bags of them in stores throughout the city and eat them however you like.