Firefox does try to make it easy to build. However they're quite opinionated and want you to use a "bootstrap" script which downloads some stuff into ~/.mozbuild regardless of you potentially already having it.

It is however still very close to a clone & bootstrap & build process, at least on Linux and Mac - i.e. one of the better projects. (For comparison, I recently tried to build Facebook's Infer, and that was even worse than building OpenOffice back in the day.)