contentScriptWhen

gistfile1.js

const tabs = require('sdk/tabs');
const { PageMod } = require('sdk/page-mod');

let s = "var x = 'start';" +
        "addEventListener('DOMContentLoaded', () => x = 'ready');" +
        "onload = () => x = 'end';";

let url = "data:text/html,<script>" + s + "</script> hey there";

let whens = ['start', 'ready', 'end'];

whens.forEach(function(when) {
  new PageMod({
    include: 'data:*',
    contentScriptWhen: when,
    contentScript: 'self.postMessage(unsafeWindow.x)',
    onAttach: function(wrk) {
      wrk.on('message', function(msg) {
        console.log(when, msg);
      })
    }
  });
})


tabs.open({
  url: url,
});