gistfile1.js

tabs = require("tabs");

widget = require("widget").Widget({
    id: "widgetID1",
    label: "My Mozilla Widget",
    content: require("tabs").activeTab.url,
    width: 400
});

tabs.on("ready", function(tab) {
  widget.getView(tab.window).content = tabs.activeTab.url;
})

tabs.on("activate", function(tab) {
  widget.getView(tab.window).content = tab.url;
});