Also sprach Bobby Holley:
> For reasons outlined in bug 1450827, the DOM peers have decided to
> deprecate support for JS-implemented WebIDL APIs. This means that
> new additions of |JSImplementation="foo"| are no longer permitted.
Out of curiosity, and mostly because my knowledge of how this works
is limited, this wouldn’t affect C++ implementations that internally
do_GetService to some XPCOM service implemented in JS?