I've been able to activate my Java VM plug-in in Netscape, but can't seem to do this in Mozilla. [...]
The simplest way is to go and download the Java VM plug-in for Netscape 6 with Mozilla. It's a large download, though, so if you're talking about Netscape 6, you could try copying the java2 and libjavaplugin_oji.so to Mozilla's plugins dir, but that might not work.
HTH