The sources for NCSA Mosaic v2.7 — one the first graphical web browsers (1993) and certainly the one that led to the World Wide Web as we know it — can now be found on GitHub.
You can even run it on a modern Linux. Here’s what the GitHub homepage looks like:
The team that built NCSA Mosaic (Marc Andreessen et al) would go on to create Mosaic Communications Corp., which eventually became Netscape Communications Corp., which open sourced the Mozilla browser, leading to Firefox.
I wonder if any of the original NCSA Mosaic code still exists in any form at mozilla.org.
The Mosaic Wikipedia entry has a thorough history.