LeaveMeAlone.user.js

// ==UserScript==
// @name        LeaveMeAlone
// @namespace   corwin.amber
// @description Prevents the leave confirmation dialog
// @include     http://*.shinezone.com/*
// @version     1
// @grant       none
// ==/UserScript==

console.log("Leave me!");
console.log(window.location);
window.onbeforeunload = null;

/* also prevent redundant horiz scrolling of the contents of the iframe */
var style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = 'body { overflow: hidden; }';
document.getElementsByTagName('head')[0].appendChild(style);

$("li.play").mousedown(() => {
  var obj = $('object');
  var parent = obj.parent();
  var html = parent.html();
  parent.empty();
  parent.html(html);
});

This script requires Greasemonkey. Install the plugin, then drag the file into Firefox.