Addon SDK code for removing onbeforeclose alerts.

main.js

function contentScript() {
  var script = document.createElement('script');
  script.setAttribute('type', 'text/javascript');
  script.setAttribute('src', 'data:text/javascript,' + escape(
      '(' + function() {
        onbeforeunload = null;
      }.toString() + ')()'));
  document.body.appendChild(script);
}

var pagemod = require("page-mod");
pagemod.PageMod({
  include: 'https://supyo.hipchat.com/*',
  contentScriptWhen: 'end',
  contentScript: '(' + contentScript.toString() + ')();'
});

You may also consider using window.defineProperty() to guarantee that onbeforeunload never gets overwritten.