The Java Plug-in 5.0 documentation has this to say about that strange behavior: “If Mozilla and Netscape 7 are both installed and <APPLET> tag support for Mozilla and Netscape is disabled (unchecked), ...

Java's unloved browser plug-in is finally being phased out. With Flash also headed for the dustbin, user security should significantly improve -- provided, of course, that people don't leave the ...