If you set Firefox as your default browser. You cannot debug a Silverlight application easily. There is a workaround for this problem.

1. Type "about:config" into FireFox'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)

Restart the browser