Whatever it is, it hasn't been touched in ~5 years. It shouldn't be in the tree.
Created attachment 584529 [details] [diff] [review]
Don't sell it short - it hasn't been touched since February 2000, so just shy of 12 years. It's the compiled version of the Rhino test driver, which used to live in http://mxr.mozilla.org/mozilla/source/js/tests/src/com/, and now lives in the past.