var box = document.createElement('hbox')
var label = document.createElement('label'); label.textContent = 'a label'
var button = document.createElement('button'); button.textContent = 'Button'
box.appendChild(label)
box.appendChild(button)

css(box, 'position:fixed')

//box.setAttribute('align', 'stretch') // не помогает

for(var i = 0; i < box.childNodes.length; i++) {
    css(box.childNodes[i], 'border: 1px solid black')
}

var panel = document.createElement('panel')
panel.appendChild(box)
gBrowser.appendChild(panel)
panel.openPopup(null, '', 700, 100, false, false)