There are quite a few tree frog species down here in Florida that could have brown coloration. If it is 3 inches or more in length and has unusually large toe pads, it's probably a Cuban Tree Frog, which is considered an invasive species in Florida because it competes with, and eats, the native tree frogs. However, as a pet, they are pretty cool frogs. But, they don't have really smooth skin. Here's a link to a photo:

http://clearwaterdailyphoto.blogspot...-frog-has.html

Here is a site that shows all the Florida native frogs:

http://www.wec.ufl.edu/extension/wil..._femoralis.php

Common tree frogs that are predominantly brown include the Pinewoods tree frog, Squirrel tree frog, and the Barking tree frog.

I hope this helps. If you post a photo, many folks on here can ID it for you.