We had a strange issue with Firefox 3.x when an exception is thrown during the execution of an onStatusChange callback during the onunload event of a page.

What happens when a page that contains an engine is closed/refreshed is that the client (obviously ) disconnects from the server generating an onStatusChange("DISCONNECTED") call.
If during such execution the callback throws an exception and the setDebugAlertsOnClientError is set to true an alert is shown.
After that alert the tab that contained the closed page will never execute any setTimeout on any page.

This should never be a problem in production environments as you should have your setDebugAlertsOnClientError set to false there, btw can be a difficult-to-understand-situation during development.