Newbie: Object Instantiation

Hello folks,

Is it true, that objects of classes used in the XPCom- environment are
always created when a JS-function is called? Or is there the
possibility to have a singleton-instance of some specific classes,
that are instantiated on startup? How do I manage this? I need a
'listener' on the application-side, that recognizes events on the GUI
(XUL). This listener should be a singleton.

Thanx for your support,
Timmy