You speak of a problem that exists in CS4 and is fixed in CS5. Unfortunately, there is no easy fix for CS4...
1) On Windows XP Professional, if you use a tiny app by Microsoft, called "TweakUI", you can set a setting that is called "Prevents applications from stealing focus". Once the setting is active, Fireworks CS4 (and older versions) cannot steal focus in WinXP, no matter what -- including batch processing.
2) But for Windows 7, there exists no such app, so TweakUI is not an option... You may try a trial of Fireworks CS5 and see if it works better for you; or you can try to find an app, similar to TweakUI, but for Windows 7.
Hope this helps!
Yes I used to use TweakUI, it was great.
I will have a look at CS5, not that I will be able to afford it for a long time! CS4 is still paying for itself