Error Console links

I have a question about the display of javascript Errors in the Firefox 
Error Console.

The Error Console will display the error when a javascript object is 
thrown (and not catched).
If this is done with the normal Error object, for example:
throw new Error("my message")
then it shows the error message and a nice link that you can use to jump 
to the file and line number
where the Error object was created.

I would like to know what the exact conditions are for this link to be 
displayed.
I ask this because I tried to extend the Error object in many ways to 
disinguish between different
exceptions, yet still have that useful link in the Error Console.

My custom Exception object is extended in such a way that "instanceof 
Error" returns true. And it contains
the fileName and lineNumber properties. Yet I still see no link.

I give some javascript to show you one method I used to extend the Error 
object:

************
clone = function (object)
{
     function Dummy(){};
     Dummy.prototype = object;
     return new Dummy();
}

extend = function (superclass, subclass)
{
     subclass.prototype = clone(superclass.prototype);
     subclass.prototype.constructor = subclass;
}

Exception = function(error)
{
     for (var p in error)
     {
         this[p] = error[p];
     }
}
extend(Error, Exception);
Exception.prototype.toString = function()
{
     return "Exception: " + this.message + " 
(filename="+this.fileName+":"+this.lineNumber+")";
}

throw new Exception(Error("my exception"));

************

Kind regards,
Robert