Firefox has way too many components to be "just one repo you can fork".

cpeterso · on Oct 10, 2023

All the code you need to build Firefox for Linux, macOS, or Windows is in one Mercurial repo: https://hg.mozilla.org/mozilla-central/

The repo is also mirrored on GitHub for convenience, though PRs are not accepted through GitHub: https://github.com/mozilla/gecko-dev

The build instructions start with a script (“mach bootstrap”) that will download and install all the blessed tools and SDKs needed to build: https://firefox-source-docs.mozilla.org/setup/index.html

Depending on your network and computer, it can be possible to download the source and compile and run your own Firefox build in less than 20 minutes.