Xlib: connection to ":0.0" refused by server ???

[daves debian <dave_arahan@yahoo.co.uk>]

When i am logged in as a user, I want to execute an X program as 
root, I type 
su
<password>

I have root access, ie I can do whatever I want !!
When I try and access an X program, like firestarter my
firewall I get ....


debian:/home/test# firestarter
NETFILTER detected

Gdk-WARNING **: locale not supported by C library
Xlib: connection to ":0.0" refused by server
Xlib: Client is not authorized to connect to Server

Gtk-WARNING **: cannot open display: :0.0
debian:/home/test#

The X library has been refused by the X server, because root is not 
autherised to connect to the server ??

Can anyone explain to a simpleton why? 
and how do I get around this ?


Any ideas
dave

PS you guys are a mine of information !!