newbie UniversalXPConnect problem

Hi all,

Apologies for the newbie question (!)

I have some code which executes

netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");

but i'm never being prompted to accept it, and the console always 
returns "Error: uncaught exception: enablePrivilege not granted".

This also happens when I try and run the XPConnect samples at 
http://lxr.mozilla.org/mozilla/source/xpcom/sample/xpconnect-sample.html

Is my syntax for UniversalXPConnect out of date? Or have I accidentally 
knocked some setting which always refuses UniversalXPConnect requests?

TIA,

phil