(In reply to Alexandre Poirot [:ochameau] from comment #0)
Bug 1524688 introduced a new way to register the XPCOM implemented
in JS statically.
I wish these sort of changes were announced more publicly. It’s
quite hard to keep track of.
Ideally we should convert to this method but unfortunately, it
breaks artifact builds support. This static definition requires
a full build of Firefox.
I always use a full build locally, so I’m not too worried about it.