Hack that executes task in a page-mod of an active tab.

active-tab-mod.js

var tabs = require('tabs');
var mods = require('page-mod');

myWidget.on('click', function() {
  mods.PageMod({
    include: tabs.activeTab.url,
    contentScriptWhen: 'ready',
    contentScript: 'new ' + function Worker() {
      window.addEventListener('message', function listener(e) {
        if ('go mod' == e.data) {
          // Remove this listener to make sure that task is executed only once.
          window.removeEventListener('message', listener, false);
          // Logic of your task goes here ....
        }
      }, false);
    },
    onAttach: function onAttach(worker) {
      // Telling active tab's page-mod to execute a task.
      tabs.activeTab.url = 'javascript:window.postMessage("go mod", "*")'
    }
  });
});