I created some entries in Netscape Communicator's address book.

Then I try to send a message.  So I netscape, choose
Communicator->Messenger from
the menu bar; this starts the mail reader.  I can read messages OK, and
send messages
using explicit addresses.

Pressing the "New  Msg" button, I get the "Compose" window.  From that, I
press "Address"
to open up the address book.  I select an entry, and press "To:" button. 
Instead of adding
that address to the To: line in the compose window...  all my netscape
windows go away!
(Even if you're not supposed to use address books this way, the result is
quite antisocial.)

I don't see an actual core file anywhere.

I get the same result after applying the glibc upgrade RPMs.