The project is hosted at http://filemanager.mozdev.org with source availabe here – there’s a server side PHP implementation as well.
Be very careful before you put this anywhere live. Right now it allows users to create and edit .php files and it may well be a good idea to review the PHP source for security.
That said think this the best example of “remote XUL” I’ve seen since the Mozilla Amazon Browser. It’s certainly more usable than any DHTML equivalent I’ve seen, considering the ability to do things like copy/cut and paste.