Extending Native javascript in dom

Hi,

I have embedded mozilla/gecko in a windows app and would like to add
some native javascript properties/functions/classes to enable the
embedded browser to be able to script at least some of the
functionality of the main program.

Here's what i came up with

nsCOMPtr<nsPIDOMWindow> outer(do_QueryInterface(GetDOMWindow()));
nsCOMPtr<nsPIDOMWindow>(outer->EnsureInnerWindow());
JSContext *cx = 0;
nsCOMPtr<nsIScriptGlobalObject> sgo(do_GetInterface(mPrivate-
>mWebBrowser));
if (sgo) {
	nsIScriptContext *scx = sgo->GetContext();
	if (scx)
		cx = (JSContext *) scx->GetNativeContext();
}
{
	JS_BeginRequest(cx);
	JS_SetErrorReporter(cx, &reportError);
JSObject * global = JS_GetGlobalObject(cx)->getParent();
	JSFunction* f = JS_DefineFunction(cx, global, "boo", &js::js_add,
2,JSPROP_ENUMERATE);
	JS_EndRequest(cx);
}

But unfortunately, when loading a webpage which contains
	<script type="text/javascript">boo(1,2);</script>
I get a javascript error.
Some-one has changed the global object in the JScontext object!
This seems to happen in mozJSComponentLoader::GlobalForLocation: every
time a new location is loaded a new global is set. Seems logical but
where can I inject my javascript objects? Is there a hook between the
setting up of the new global object and the actual loading of the web
page?

Many thanx!

Kenny