The only issue is that you have to clone whole mozilla-central even if you only work with small part of it. But once you pull it, the building and hacking on it is quite easy :)
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.)
If you do want to tweak the native code it should just be ./mach boostrap + ./mach build, but it's entirely possible you'll have trouble with that.
At least that is how it feels to me after having spent some years using a distro that is a fair bit more hands on than most.
The worst offender I've encountered has been android. The initial checkout is like +100GB, and it has incredibly poor fault tolerance. Based on my experience, it fails regularly at random points, requiring you to re-fetch everything. Despite multi-day efforts, I've never managed to build a ROM for my Nexus 5X or Pixel.