>Why make your own build?
To build for x86-64, when Mozilla doesn't give binaries.
To enable/disable certain build flags (features)
To build with optimizations for your machine (not very useful with FF I think).
>Is it like creating a distro, just choosing default packages and libraries and stuff?
I have no idea what are you talking about. You download the source and run a few terminal commands, or maybe load into VS and press F5.
Go to Mozilla's build guide and read it, I think.
I build with makepkg, so I have it easy, but I had problems with pgo builds too, I needed to build in chroot.