Netscape 6 was not based on the code from Netscape 4. Instead the developers decided to start over from scratch and develop completely new code that would be as compatible as possible with the web standards. At the same time they decided that they would take advantage of the world's programmers by producing an open source version of their browser. This open source version was called Mozilla.
The fact that Mozilla is open source means that anyone who can program can download the source code for the browser and create their own modified version that adds the features that they want. They can then make their modified version available for others to use. Those modifications that prove popular and useful will eventually find their way into the base version of Mozilla and perhaps from there into a subsequent version of the Netscape browser.
The Netscape browser is based on Mozilla but again there are differences with features that have been both added and removed compared to the open source version.
A number of other browsers have been developed based on the Mozilla rendering engine (called Ghecko). The main one developed by the open source community was originally called Phoenix and then renamed FireBird while is was still in development. The final version that was released for general use was called Firefox and it was the first browser to challenge Internet Explorer's dominance of the browser market that it had gained when development of the original Netscape beyond version four ceased.
This article written by Stephen Chapman, Felgall Pty Ltd.