问拖动textarea firefox拖动面板

EN

var win = Services.wm.getMostRecentWindow('navigator:browser');
var panel = win.document.createElement('panel');
var screen = Services.appShell.hiddenDOMWindow.screen;
var props = {
   noautohide: true,
   noautofocus: false,
   backdrag: true,
   level: 'top',
   style: 'padding:10px; margin:0; width:530px; height:90px; background-color:rgba(0,0,0,0.3); border:none;'
}
for (var p in props) {
   panel.setAttribute(p, props[p]);
} 

var textarea = win.document.createElement('textarea')
textarea.disabled = true;
textarea.readonly = true;
textarea.overflow = 'hidden';
textarea.width = '525px';
textarea.height = '85px';
textarea.style.textAlign = 'center';
textarea.style.fontFamily = '"New Century Schoolbook"';
textarea.style.color = 'white';
textarea.style.fontSize = '21px';
textarea.style.fontWeight = 'bold';
textarea.style.fontStretch = 'semi-condensed';
textarea.style.backgroundColor = 'rgba(0,0,0,0)';


panel.appendChild(textarea);
textarea.innerHTML = 'text';

win.document.querySelector('#mainPopupSet').appendChild(panel);

panel.addEventListener('dblclick', function () {
   panel.parentNode.removeChild(panel)
}, false);

panel.openPopup(null, 'overlap', screen.availLeft+screen.width/2, screen.availTop/2);