xpconnect garbage collection

[Alex Fritze]

Hi,
I don't know whether this an xpconnect or js-engine question.
I noticed that when you create an object in js through xpconnect, e.g.

 var inst = Components.classes["@my-company/my-class;1"].createInstance();

it never gets garbage collected until the current document is deleted, even
if 'inst' goes out of scope or is explicitely set to zero.
Is this by design? If yes, is there a way to force deletion?

Thanks
alex