Fireworks is not very compatible with El Capitan, and a number of issues exist. Remember, Fireworks was discontinued in May 2013, and is dead software, no longer supported, and no bug fixes or updates are forthcoming from Adobe.
Officially Fireworks CS6 is not compatible with the latest OSX.
Unofficially you might be able to get it to run by installing a clean El Capitan, install legacy Java (which is insecure, btw), and reinstall Fireworks.
However, reports in getting Fireworks to run on El Capitan are mixed, with some users stating they are successfull, other failing, and yet others having Fireworks (and other CS6 apps) crashing randomly.
More info related to Java here: http://www.macstrategy.com/article.php?135