FireFox : Debugging Problem in Silverlight

Solution 1:

Manually attach the visual studio debugger to “plugin-container.exe”

Solution 2:

This involves changing Firefox’s config, and really should only be done to ease silverlight debugging until a proper fix is released.

1. Type "about:config" into FF's address bar
2. Accept the warning (if applicable)
3. Search for the entry "dom.ipc.plugins.enabled.npctrl.dll"
4. Change its value from "true" to "false" (double-click)
5. Restart the browser

Thanks MisterGoodcat for this solution  More info