I did another search and found some hidden java plugins from past years. I got rid of those and there is definitely no java now. If I already had some old java, I guess that was not good enough for some newer applications that insist that they need java to run.
I guess Mozilla has its own ability to unarchive .jar files, then (and so can my favourite archive handler). Obviously, it has to handle .ja files (I have to do it from the Mac Terminal).