https://www.virtualbox.org/wiki/Developer_FAQ > *Q*: Why do you ship source codes from Mozilla and build them as part of VirtualBox? This increases build times and every sane person on this planet already has Mozilla Firefox installed! > *A*: On Linux hosts, VirtualBox makes use of Mozilla XPCOM as its component model. Unfortunately, XPCOM is not very standardized and each and every application based on it brings its own binary version of XPCOM. Also, we have significantly enhanced XPCOM for cross process communication and our enhancements have not been incorporated into the Mozilla code base yet. We therefore cannot rely on XPCOM being present on our target systems and even if, it would not have our enhancements. For Windows hosts, we do not rely on XPCOM so we don't build it there.