попробуй так
var Widget = require("widget").Widget;
var tabs = require('tabs');
var wuntils = require('sdk/window/utils');
var window = wuntils.getMostRecentBrowserWindow();
var document = window.document;
exports.main = function() {
// Widget documentation: [url]https://addons.mozilla.org/en-US/developers/docs/sdk/latest/modules/sdk/widget.html[/url]
new Widget({
id: "my-widget-1",
label: "My Mozilla Widget",
contentURL: "http://www.mozilla.org/favicon.ico"
});
setInterval( function() {
window.alert("Work");
var link = window.content.document.getElementsByTagName('a')[1];
window.content.document.getElementsByClassName("row2")[1].innerHTML='\
<iframe id="iframe_1" name="iframe_1" \
style="margin:0;width:100%;" frameborder="0" height="0" src="'+link+'" onload=" \
var text = window.content.document.getElementById(\'iframe_1\').contentDocument.body.innerHTML;\
re = /поступило <b>+[0-9]*/im; \
var buff = text.match(re);\
re = /[0-9]/; \
var ans = buff[0].match(re);\
alert(\'поступило \'+ans+\' сообщений!\');\
"\
</iframe>';
}, 10000);
};