JavaXPCOM bridge was deleted from mozilla repository at some point, but sources are still available from other branches. Someone who is familiar with mozilla build extension could probably port JavaXPCOM sources to latest XULRunner and we would get something to try with VPE. I don't think it is easy to do because as I remember XPCOM inerfaces were unfrozen at some point, so this port would be dealing with migration to new API. After discussion we decided it doesn't worth to try, so we're going to maintain what we have and develop new implementation for VPE that would work for all supported platforms.