Check old component before hiding it for a component switching. Ref: ticket #37 in assembla - Scribboo try to hide old component before switching to new one. But, old one was already removed. It cause an error. - This is fixed by checking old component if it is still existed before hiding it.

function start_madbuilder() {
    var url = "chrome://MadBuilder/content/madbuilder.html";
    var browser = document.getElementById("content");

    browser.loadURI(url, null, null, true);
}