Huh, I was curious about how that was done, and it looks like it's a specialized Java to C++ converter:
https://github.com/validator/htmlparser/blob/master/translat....
Looking at the source, it seems pretty easy to find Java programs that wouldn't compile correctly, but of course a fully general Java to C++ converter would be a huge undertaking. I guess with OK test coverage of the generated C++ code (which Firefox certainly has), editing the Java code remains doable.
