Unfortunately, I had fully missed that information, but I managed to update Firefox, that was the only real problem.
But, is it possible to update the existing installation from a new ZEN-mini live CD without re-installation - as I saw in the past with Fedora - or just any particular software using another Live, technical Distro especially intended for that task (like Clonezilla, for instance: I don't know if it's the right one, I just remember that name) ?
No.