Hi,
I live in Florida (Orlando). In December I usually wear a thin long sleeve shirt with a thin jacket and pants and it is enough for me (I do get hot easily though). Depending on where you want to visit the weather may vary. If you are thinking about Miami, it is quite a bit hotter. I have a friend from Boca (south Fl) and she said it was 80 degrees F one Christmas. So you probably want to check out the weather when it is closer to Dec.
Where in Florida are you thinking about visiting?Just let me know if you need more help.