XPCOM is going to change, and in some cases change quite radically: we're
dropping all pretense of binary compatibility between major releases in
order to improve performance characteristics, and we're likely to eventually
change to a garbage-collected runtime.
Since the JS representation of objects is the one the web sees and the one
that is actively QAed, I really strongly suggest that future bridges work on
the JS level: this is what the NPAPI does already in the Sun Java plugin; I
don't know whether it's possible to use that same code in Eclipse or not,
but that's the kind of API you need to end up with, if you want stability.