http://www.xulplanet.com/tutorials/xultu/broadob.html http://www.xulplanet.com/reference/elemref/ref_observes.html The observes element can be used to listen to a broadcaster and receive events and attributes from it. The observes element should be placed inside the element that wants to observe the broadcaster. When an observed attribute is modified on the broadcaster, the attribute's value will be forwarded and set on the parent element of the observer. The attribute that the observer is observing. When the value of the attribute changes, the broadcast event is called on the observer. Use the value '*' to observe all attribute of the broadcasters. The id, ref and persist attributes are not observed. The id of the broadcaster element that the observer is observing.