Thanks BostonBoy, that link helped immensely. The following dll's were missing from the plugin folder:

* npdsplay
* npwmsdrm
* npdrmv2

I installed them in the Firefox plugin directory and everything worked fine. So the question now is this: If these plugins are needed, why aren't they installed when you install WMV Player? Am I too much of a conspiratist to suggest that they're not installed automatically so people using Firefox will get pissed and go back to IE?

Thanks
Michael