« XUL Reference
Type: string
Sets the label attribute. Gets the label attribute if it is present and not empty. Otherwise it returns the value of the associated label element, if applicable. Otherwise it returns the placeholder or emptyText property. The getter is mostly useful for screen readers.

Note: Prior to Firefox 3, and always in Thunderbird and SeaMonkey, the label property of an autocomplete textbox returns its value, for compatibility with the menulist element.