If you've Firefox 3.6.4 or above installed on your computer and if you use it as your default browser, you'll probably meet some problems while trying to debug a Silverlight application in Visual Studio.

The Mozilla team has changed security settings for plugins.

I found the solution in the Silverlight forum (Thanks to MisterGoodcat!).

Here is what you to do :

• Type "about:config" into Firefox address bar
• Accept the warning (if applicable)
• Search for the entry "dom.ipc.plugins.enabled.npctrl.dll"
• Change its value from "true" to "false" (by double-clicking it)
• Restart the browser