How can I install Web Start on Unix as root once for all users?
Brett Humphreys solved the puzzle. Web Start updates the current users’ .mime.types and .mailcap files. However, if you install Web Start as root (that is, once for all users), it won’t update netscape’s global files, so you have to do it yourself. Here are Brett Humphreys’s distilled instructions to register Web Start as Netscape’s helper app for JNLP mime types as root once for all users.