Linux Mac OSX Windows 
 The nsIDOMBeforeUnloadEvent interface is the interface for events
 sent to handlers of the "beforeunload" event. This event is
 non-standard. Interface derived from Microsoft IE's event
 implementation.

 http://msdn.microsoft.com/library/default.asp?url=/workshop/author/dhtml/reference/events.asp
[scriptable, uuid(da19e9dc-dea2-4a1d-a958-9be375c9799c)]
interface nsIDOMBeforeUnloadEvent : nsIDOMEvent

Attributes

 Attribute used to pass back a return value from a beforeunload
 handler
attribute DOMString returnValue