T-Bird Extension How-To?

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I am interested in writing a T-Bird extension that is triggered every
time that T-Bird discovers that new mail is available (even in not
actually downloaded).

In my Windows T-Bird, I see a popup displayed when this happens. Does
any one know (and ideally can provide some js code sample of) how my
extension can get these same notifications?

I have looked at code for existing extensions like MailAlert and Yamb
but these appear to monitor the mail folders and therefore only get
triggered if/when mail is downloaded. So this is not really what I want.

Thanks in advance.