I assume you mean sign with S/MIME. You cannot. Netscape left it out of 6.0. I hear that they intend to put it back in 6.next. You can stick with 4.7 of Messenger (which is what I did) and wait for 6.next with S/MIME, or you can purchase an add-on product, although I don't know of any specifically that work with Netscape.
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