Find URL of current tab. Making a FireFox Browser add-on

I'm making a Firefox Browser Add-on and need to find the url of the current tab

I've tried this post Opening a URL in current tab/window from a Firefox Extension but it tells me that 'window' is not defined. (I think because I am making an add-on rather than an extension.)

Here's what I've tried to do:

var widgets = require('widget');
var tabs = require('tabs');

var widget1 = widgets.Widget({
  id: "widget1",
  label: "widget1",
  contentURL: "http://www.mozilla.org/favicon",
  onClick: function() {
    console.log(tabs.url);
  }
})

I've made a widget such that when I click it the url of the current tab should be 'console.log'ed.

Doesn't seem to happen! Keep getting "info: undefined" which clearly means that tabs.url isn't returning anything. But this seems to be the way to use it according to https://addons.mozilla.org/en-US/developers/docs/sdk/1.5/packages/addon-kit/docs/tabs.html

Anyone have any ideas?

Thanks,

Will