IMO, CentOS is not a good choice for old hardware with small amounts of ram, esp since the apps you want to use (Fx2 and OoO) are notorious for being profligate with ram. If no-one here can point you to a Redhat-based distro with modest requirements, you could check out Zenwalk, which uses XFCE. It is a slackware-based distro and is famous for including only one app for each task and running like the blazes on old hardware. It is not a cutdown semi-useable distro like DSl and puppy.