nsIDOMBeforeUnloadEvent

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

Attributes

returnValue

Attribute used to pass back a return value from a beforeunload
handler