If the news server is down or unreachable, nntp protocol doesn't get notified of the error, and thus never tells the user. ::OnStopRequest gets an NS_OK status. 4.x puts up an error message when trying to connect to the same server.
Seth thinks there might have been a recent check in necko that might be responsible. My release build from a few days ago has this problem, so it has been around for at least 4 days.
you might try backing out my most recent checkin to nsSocketTransport.cpp
*** This bug has been marked as a duplicate of 114328 ***