The power of symlinks is fine if you can create symlinks - however, what's happening is that ftp.mozilla.org won't be a mozilla server (this is clearer in the original post, behind the full article link), it will point to a bunch of mirrors, and you can't create symlinks in the root folder of other people's mirror machines.
With http, you can do different things based on the hostname used to access the server (at least you can if the client machine isn't using netscape 2.0 or something else ancient). With ftp, the client doesn't tell the server what name it's using, so you can't do the vhost trick.
ftp.mozilla.org Moving Servers Next Wednesday