Fireworks is not "quite" compatible with El Capitan, and the situation will only get worse (FW's last update was in May 2012, and development was stopped in May 2013)
Did you install any Mac Os updates last week?
Thank you for your answer.
My Mac is completely updated so yeah! This is such an annoyance! Have always favoured Fireworks over Photoshop, and i knew they stopped developing it, but not that its being effectively de-mobilised haha!
So would there be no way for Fireworks to work decently on the new running system?
It might have to do with a missing Java version 6, which is required for Fireworks and other CS6 products to run properly. However, the older Java version (6) is no longer supported by Oracle, and has several known security holes. OSXDaily warns that:
To be clear, Java 6 is a version from 2013, it is outdated and no longer supported by Oracle, it has various known security holes, and is therefore inappropriate for most Mac users to install without a compelling reason. So, unless you have a need for Java 6 specifically, you’d likely want to get a newer version, or if you don’t need Java at all, just don’t install it.
Reference: How to Install Java in OS X El Capitan
But you do have a compelling reason: you wish to run Fireworks CS6.
Word of warning: reports from other users trying to install Fireworks CS6 on El Capitan range from failures to working installations "with caveats". Even after a successful installation there will be some issues.
Although Fireworks does not work that well anymore on the newer Mac machines, it does work on Windows machines: perhaps consider Parallels, or VirtualBox to install a virtualized Windows environment on your Mac to run the Windows version of Fireworks instead?
You can run Windows 10 for free on your Mac with VirtualBox. Then install the Windows version of Fireworks. See: How to Run Windows 10 on Mac Free with VirtualBox for OS X
Parallels is faster, though commercial: KB Parallels: How to Install Windows 7 in Parallels Desktop 10 for Mac
I read that it even works in WINE under Mac - which can be installed on your Mac, although it involves a fair bit of technical know-how to install.