Standard main install locations are /usr,/usr/local and /opt, use
when in the mozilla-relase folder to get a list of options, the install prefix for most autotools programs is /usr/local, your distro will tend to use /usr, this keeps your compiled stuff away from the official install, if however you use /usr as the prefis you will ( probably ) overwrite the firefox that has been installed by your distro, this may or may not be what you want as any updates by your package manager will overite ( probably ) your custom installed firefox.