_ff-addon-snippet-OpenNewTabGetIframeHtml - This snippet must run in privelaged scope. It opens a new tab and messages you when it loads, and it looks for all iframes in that new tab and messages you when iframes contentDocument is ready and populated. I do need a better way to test if iframe is loading/loaded on initial run of the getIframeHtml…

_ff-addon-snippet-OpenNewTabGetIframeHtml.js

Components.utils.import('resource://gre/modules/Services.jsm')
var aDOMWindow = Services.wm.getMostRecentWindow('navigator:browser');
var newTabBrowser = aDOMWindow.gBrowser.getBrowserForTab(aDOMWindow.gBrowser.loadOneTab('http://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_tolocaletimestring', {
    inBackground: false
}));
newTabBrowser.addEventListener('load', function onloadFunc() {
    Services.appShell.hiddenDOMWindow.console.log('tab loaded');
    newTabBrowser.removeEventListener('load', arguments.callee, true);
    var doc = newTabBrowser.contentDocument;
    var win = doc.defaultView;
    var iframes = doc.querySelectorAll('iframe');
    Services.appShell.hiddenDOMWindow.console.log('number of iframes in this tab = ' + iframes.length);
    [].forEach.call(iframes, function (frm, i) {
        logIframeHtml(frm, i);
    });

}, true);

function logIframeHtml(iframe, i) {

    iframe.removeEventListener('DOMContentLoaded', logIframeHtml, true); //this line removes the event listneer if it had one

    var iframeDoc = iframe.contentDocument;
    if (!iframeDoc) {
        Services.appShell.hiddenDOMWindow.console.log('iframe ' + i + ' is not fully loaded yet as it its contentDocument does not exist');
        iframe.addEventListener('DOMContentLoaded', logIframeHtml, true);
    } else {
        var iframeInnerHtml = iframeDoc.documentElement.innerHTML;
        Services.appShell.hiddenDOMWindow.console.log('iframe ' + i + ' innerHTML = ' + iframeInnerHtml);
    }
}