So I wanted a little popup Biff notification to appear when feeds have been updated. After asking in the Mozillazine forums the answer turned up. Courtesy of richwklein, here you go:
var alerts = Components.classes["@mozilla.org/alerts-service;1"].getService(Components.interfaces.nsIAlertsService);
alerts.showAlertNotification(aImageUrl, aAlertTitle, aAlertText, alertTextClickable, aAlertCookie, aAlertListener);
Every argument to showAlertNotification can be null, then you want alertTextClickable as a boolean and aAlertTitle/aAlertText as a string. Lovely!