the readme is literally 21 lines long, and about 25% of them are blank lines, with the 2nd URL pointing to "how to contribute" and that link has a dedicated "how to build and run" https://firefox-source-docs.mozilla.org/contributing/contrib...

The bad news is that their build system is extremely hand-rolled, and so if it works for you, count yourself lucky, because when it doesn't work you're in for 4 hours of python hell