Firefox is relatively secure and well designed web browser. Any closed source plugin and especialy the Windows Presentation Foundation Plugin from Microsoft can be serious security hole. There is also posibility of your privacy violation. Who knows what the Windows Presentation Foundation Plugin from Microsoft exactly do and what data it reads, colect and process ? Generall security rulle is not to use any software with closed source, becouse it cannot be verified therefore it cannot be trusted.
Okay so I got a pop-up from Firefox saying it is blocking Windows Presentation Foundation as it’s unsafe. In the same light as the .Net Assistant, the WPF can not be uninstalled with a GUI. Firefox blocks it now and if you want to remove it, follow the link above. I think it probably piggy-backed on the slew of security updates released a few days back.