this is kind of tricky: I have an C# web application running always on http://localhost (it is configured like this in csproj properties).
The site runs fine on VS 2008, but Firefox plugin called Firebug is showing this messages:

The files exist on the folders, but IIS has some trouble when trying to show them.
What can I do to solve this issue?
My environment: IIS 7, Windows 7, Visual Studio 2008, Firefox 3.6. Classic Application pool.
This problem was solved when bypassing Proxy configuration for localhost :-/