Except that Netscape was open sourced years before it became Firefox.
That happened January 1998, which was 17 years ago, and 4 years before Firefox was released, which itself was over 12 years ago.
March 1998, and it underwent a pretty well-publicized rewrite as well... Firefox is not built on top of what was released back then (Communicator 4.x); it's built on top of its rewrite.