Hey it's Firefox, one of the biggest C++ project you can build for now.
A modern browser is a MUCH larger codebase.
Well perhaps if browsers didn't try to cram every feature of a desktop operating system into their own runtime, it wouldn't be?
Seriously. Chrome and Firefox are both guilty of this.
I've recently built php + most of the bundled extensions, in a VM (on a 2018 Mac mini) and it's only a couple of minutes at most to build.