The date on your message is months ago...I hope you were able to solve your problem. I am currently trying to get any app to start on a specified virtual desktop. Sounds like maybe you were able to do this, just Firefox was giving you trouble. Any pointers you might be able to share with respect to referencing different screens (4 headed workstation) and their respective virtual desktops within a start script would be greatly appreciated.
I'm still very new in the KDE customization, but your response will at least get me started looking in the right direction.
Cheers,
Tim