Pretty major redesign here, so: He's wearing a turtleneck that's way too big for him, and thus the sleeves are pushed up to about his elbows. There's a black shirt underneath. It's a little hard to see, but there's a sort of stylized flame symbol on the front of his sweater. I decided to just have his eye stitched shut, because I have too many one-eyed characters so all the eyepatches were taken. Also it looks less ostentatious I guess. I don't know what he's wearing below the waist. Firefly's tail doesn't usually stick up like that; I just felt like making it visible.
For cinaparadox, who kindly offered to make a talksprite of Firefly. Drawn with GIMP, brutally murdered by Scratch. Song is Frost, by Homestuck. It's the most fitting thing I've got, but if you've (yes, you, CP) got anything better feel free to use it.