Why does JavaXPCOM require XULRunner?

Hey guys,

This question has probably been asked a million times, but after searching
the archives I didn't find anything: Is it possible to use JavaXPCOM to make
Java apps communicate with Firefox extensions, or is it truly
XULRunner-only?

I made a sidebar Firefox extension to act as the GUI for a command-line Java
app I wrote. The only problem is that the communication is one-way -- I can
use nsIProcess to launch the .jar file from the extension, but it can't
receive any information back as the app is running!

Thanks very much.