Let me start by saying that I enjoyed this little short story. Mitosis, though short, is an essential read if you plan to continue past Steelheart. Sanderson really shows his ability to write action in a way that makes your mind play the scenes unfolding as if on the big screen.
After the fall of Steelheart, the Reckoners have had to add administrative roles to keep Newcago running and the people safe. They have to do this while also stopping the Epics that have flocked to the city to stake their claim. The Epic that this story revolves around is Mitosis. As the name implies, he has the ability to split into many versions of himself and swarm the city.
Though the pacing was great, I think Sanderson should have either expanded the story, or built this into Firefight, possibly as a prologue. A minor misstep, but I still recommend readers of the series pick up a copy before reading Firefight.