XmlHttpRequests that initiate from code executed in an iframe, that is then destroyed, result in an error in FireFox. This test case is used to verify that Closure's IframeIo and XhrLite do not suffer from this problem. See https://bugzilla.mozilla.org/show_bug.cgi?id=369939.
NOTE(pupius): 14/11/2011 The XhrMonitor code has been removed since the above bug doesn't manifest in any currently supported versions. This test is left in place as a way of verifying the problem doesn't resurface.