Error "Server Application Error" al teclear http://127.0.0.1/

[B**]

Yo tengo un error parecido cuando tecleo la direccion ip/prueba.php en firefox, me sale esto:
Network Error (tcp_error)

A communication error occurred: ""
The Web Server may be down, too busy, or experiencing other problems preventing it from responding to requests. You may wish to try again at a later time.

For assistance, contact your network support team.

Y al teclearlo en explorer funciona perfectamente..
Q podra ser?
Gracias de antemano