As you are aware, the development of Fireworks was halted in May 2013. There will be no updates forthcoming. Support is no longer given. Fireworks is dead in the water, and it is to be expected that more and more compatibility issues on newer operating systems will pop up.
Sad, but true.
I suspect as much but there is always hope. At least the response I got form Adobe's customer service on Twitter was to post here because the developers would take care of it...
Anyway if there is no way to make it work in El Capitan that is going to be a big problem since the design studio I work for refuses to use anything but Fireworks CS6 for their designs. There aware that Fireworks is dead, but they just don't care. That means that the developers who work with them must keep our systems out of date just to be able to open their Fireworks-PNG files. It drives me nuts.
I might need to look at downgrading to Yosemite again.
I feel for you - you are between a rock and a hard place. Fireworks seems to behave much better on even the newer Windows systems: perhaps consider installing a Windows XP virtual machine on your Mac, and run Fireworks (windows edition) in it instead.