MOZ_FS_LAYOUT is a makefile variable, set by configure and autoconf.mk, which determines which style of build is being produced. There are only two valid values:
unix- Build files are produced in dist/bin and libxul is named using the standard shared-library naming scheme (libxul.so or xul.dll)
bundle- Build files are produced in dist/AppName.app and libxul is named as if it were part of a framework (XUL)
This variable exists mainly to distinguish the standard mac builds, which use the cocoa toolkit and bundles, from the Darwin/X11 builds, which use the unix structure.