Comment 4 for bug 113201

yeah, the problem is that IsInNet function needs to synchronously resolve the
given hostname, so Mozilla ends up locking up the main thread (the thread which
handles browser UI) until the DNS query completes. Since DNS queries can have
relatively large timeouts (on the order of a minute), this can make the user
think the browser has completely locked up. Unfortunately, there is little we
can do about this problem, with the exception that it would be nice if
synchronous DNS queries could be canceled by the user. We could put up a modal
dialog with some sort of status message and a cancel button (if the DNS query
was taking more than X seconds). Such UI work isn't going to happen for
sometime unfortunately.