Mozilla is a Web browser developed by the Free Software Community, with the cooperation and support of Netscape.
The build system is fully automated. You have to set up a build environment by assembling various pieces, but after that, you can check out and build all projects with a single Perl script.
The build system is still rapidly evolving, and is not idiot-proof, but it is usable.
For more detailed information, including on updates, please read the documentation or visit the home page.