Plugin is not able to get notified about 'http-on-modify-request' (c++)

Hi All,

I am writing a sample plugin which will get notified whenever http
request is made. Below is a information about the environment-
Programming language - C++
Gecko / XUL runner - 10.0.2
Firefox - 10.0.2
OS: Windows Vista.

Below is how plugin registers for 'http-on-modify-request'-

	nsresult rv;
	nsCOMPtr<nsIObserverService> observerService =
do_GetService("@mozilla.org/observer-service;1", &rv);
    if(NS_FAILED(rv))
    {
        return false;
    }

    rv = observerService->AddObserver(m_ptrEventHandler, "http-on-
modify-request", false);
    if(NS_FAILED(rv))
    {
        return false;
    }

This is how I am handling observer

EventHandler.h
class CEventsHandler : public nsIObserver
{

NS_DECL_ISUPPORTS
NS_DECL_NSIOBSERVER
CBrowserEventsHandler()
{
}

}

EventHandler.cpp

NS_IMPL_ISUPPORTS1(CBrowserEventsHandler, nsIObserver)

NS_IMETHODIMP CEventsHandler::Observe(nsISupports *aSubject, const
char * aTopic, const PRUnichar * aData)
{
//Handling observers
	return NS_OK;
}


Please let me know if more information is needed.

Any help / sample code / pointers would be a great help.

Thanks and regards,
Umesh