Not my product, but I believe I've been in this argument before, and went to check the Firefox source code, and found files that were in the original Netscape code base. So I'm going to say: no it wasn't a from-scratch re-write even for the Firefox fork. The actual 1998 open source event was totally the Netscape code, fwiw, with some stuff removed to comply with commercial licenses, build process decoupled from the corporate network and so on.