That information is outdated; I build FF from source a couple of times a week, and use python3.9 for mach's python. I do agree the build is special needs, made worse by all the rust, but of the things I really hate about Firefox, the build is pretty low on that list

The Chromium (and its derivatives) builds makes it clear they have a compile farm of unlimited compute and ram, so it's just "which kind of bad" I nowadays, I guess