How to get currently selected tab index and its content

Hi,

I am developing an XPCOM component in c++. I am able to get number of
tabs currently opened using XBL. Please see the code below:

////////////////////////////////////////////C O D
E//////////////////////////////////////////
nsresult rv;
nsCOMPtr<nsIWindowMediator> windowMediator
= do_GetService(NS_WINDOWMEDIATOR_CONTRACTID, &rv);
NS_ENSURE_SUCCESS(rv,rv);
PRUint32 len;
nsString temp;

nsCOMPtr<nsIDOMDocument> curDoc;
nsCOMPtr<nsIDOMElement> domEl;
nsCOMPtr<nsIDOMNodeList> tabbrowser;
nsCOMPtr<nsIDOMDocument> doc;
nsCOMPtr<nsIDOMWindowInternal> dwi;
windowMediator->GetMostRecentWindow(L"navigator:browser",
getter_AddRefs(dwi));
if (dwi)
{
	dwi->GetDocument(getter_AddRefs(doc));
	if (doc)
	{
		nsCOMPtr<nsIDOMDocumentXBL> xbl(do_QueryInterface(doc));
		if (xbl)
                       return NS_ERROR_FAILURE;
		doc->GetElementById(NS_LITERAL_STRING("content"),
getter_AddRefs(domEl));
		if (domEl)
		{
			nsCOMPtr<nsIDOMElement> pAnoEl;
			xbl->GetAnonymousElementByAttribute(domEl,
				NS_LITERAL_STRING("anonid"), NS_LITERAL_STRING("tabcontainer"),
getter_AddRefs(pAnoEl));
			if (pAnoEl)
			{
				nsCOMPtr<nsIDOMNodeList> nodeList;
				pAnoEl->GetChildNodes(getter_AddRefs(nodeList));
				if (nodeList)
				{
					nsCOMPtr<nsIDOMNode> domNode;
					nodeList->GetLength(&len);
					for (PRUint32 i = 0; i < len; i++)
					{
						nodeList->Item(i, getter_AddRefs(domNode));
                                                curDoc =
do_QueryInterface(domNode); // fails here.
					}
				}
			}
		}
	}
}

///////////////////////////////////END OF
CODE////////////////////////////////////////

Here I am getting number of tab, even I am able to set/get attributes.
But I am able to decide which is the currently selected tab. I tried
to use property and method from tabbrowser.xml, but failed.

Also I want to get the content of currently selected tab. But I tried
to QI nsIDOMNode for nsIDOMDocument, it failed. But in dependency
diagram on mozilla site, it is given that nsIDOMNode is derived form
nsIDOMDocument.
How can I get the current tab content, DOM pointer and its index?
Please help me, I am stuck to it.

Thank you in advance.