From my vague understanding of the history, Mozilla code goes all the way to Netscape. I think Netscape did a giant rewrite, so it probably doesn't go back further.

Technically, firefox is the browser front-end to the Mozilla codebase, and Thunderbird is another front-end of the same product. I think what used to be called the Mozilla suite is now seamonkey. So I think it all comes from the Netscape release.

Certainly, whenever I touch the build system, the copyright notices are from 1998, say Netscape and mention jwz. Example: https://mxr.mozilla.org/mozilla-central/source/configure.in