The Pkcs11 object is part of the security model built into Netscape Navigator. It is otherwise known as Cryptoki and is provided by RSA Data Security, Inc.
They implement a C language API that has now been mapped to Java as well. According to the Netscape web site, it is not a fully-fledged object oriented API, but can be readily understood by programmers already familiar with Cryptoki.
According to the release notes, Netscape 4.04 added support for the FORTEZZA PKCS#11 module for making use of the FORTEZZA Crypto Card and FORTEZZA cryptographic algorithms (KEA and Skipjack) when using SSL and S/MIME. Although a link was provided for more details, it appears that the support documents may have been moved or deleted.