The projected release date for Firefight is fall of 2014, but if you are itching to get back into the world of Steelheart, I would suggest reading Brandon Sanderson's novelette, Mitosis: A Reckoners Story. At a mere 35 pages, Mitosis is a quick read that takes you back to dystopian Chicago with the Reckoners and a new Epic.
As a caution, there are definite spoilers in these chapters, so read Steelheart first to set up the characters. With just enough details to refresh your memory, this story provides a nice gateway to the series sequel and the inventive new Epic reminds you what was great about Steelheart. The only thing that would make this better? - not having to spend $1.99 for the five chapter story. This seems like the perfect release as a fan-favorite bonus gift. But, in the meantime - definitely a fun read and a nice companion to Steelheart.
Mitosis: A Reckoners Story
by Brandon Sanderson
published by Delacorte Press (as an eBook)