add: var button = document.getElementById("changeMasterPassword"); button.disabled = noMP; 
context:   _initMasterPasswordUI: function ()  {    var noMP = !this._masterPasswordSet();    //var button = document.getElementById("changeMasterPassword");    //button.disabled = noMP;    var checkbox = document.getElementById("useMasterPassword");    checkbox.checked = !noMP;  },
meta: 10234	/local1/tlutelli/issta_data/temp/all_js/javascript/2007_temp/2007/10234/d30bd593f72f099cf7736a11485309a0c1a9a1e3/security.js/buggy/browser/components/preferences/security.js