I'm getting the same exact issue. Highly annoying. I prefer working in Fireworks, but my design team is on Photoshop.
I'm doing a ton of design work right now and would love a solution to this.
Older software can be expected to run into compatibility problems when it is run on newer operating systems. You write it works in CS6 - why not update to CS6 then? If it is essential to your workflow and team, it seems a small price to pay.
Alternatively, turn off every single other application and background service that you can afford to, and try again. Fireworks is known to have issues with certain newer system utilities under MacOs.
My boss won't upgrade sadly. It's not a massive issue as I can jump on another computer and save out. Just annoying as I need to save out to PSD quite regularly.
Its one of those bugs that I guess Adobe don't want to concern themselves with anymore. At least it works in CS6 so all is not lost.