Because OS-tan fanart is much fun. Not to mention that I love Firefox-tan's design.
I decided to throw some quick computer coloring on the scanned lineart even though I'm such a n00b at computer coloring, but I don't have any orange Copic Markers either so didn't have much choice. I gotta admit that I actually rather like the outcome, and that doesn't happen often. o_o The brush used for the background was made by Iroka.