Петр

overlay chrome://browser/content/browser.xul chrome://vkwall/content/browser.xul

browser.xul

<?xml version="1.0"?>
<overlay xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">  
    <script type="application/x-javascript" src="main.js" />
    <menupopup id="contentAreaContextMenu" oncontextmenu="">
        <menuitem id="saveToFile" label="Отправить на стенку" oncommand="newTabb();"/>
    </menupopup>
</overlay>

main,js

function newTabb(){
		var params = "menubar=no,location=no,resizable=no,scrollbars=no,status=no,width=600,height=540";
		var img = (content.getSelection().focusNode.firstElementChild.src != undefined) ? content.getSelection().focusNode.firstElementChild.src : "" ;
		content.window.open("http://vk.com/share.php?image="+img+"&title="+content.document.title+"&url="+content.location.href+"&description="+content.getSelection().toString(), "Отпраить на стенку", params);	
}