- Check Firefox build prerequisites for the OS you will be using. I was using Windows 7. *[https://developer.mozilla.org/En/Developer_Guide/Build_Instructions/Windows_Prerequisites Windows Prerequisites] - Download the latest [http://ftp.mozilla.org/pub/mozilla.org/mozilla/libraries/win32/MozillaBuildSetup-Latest.exe MozillaBuild package]. This package contains other software that will be needed to successfully build Firefox.