Ironically Firefox killed their extension API to be compatible with Chrome's extensions. Now they probably don't want to be compatible with the new Chrome API.
This is misleading. They deprecated the old API primarily to be able to do multi-process and get rid of XUL.
Its sad to lose DownThemAll and the plethora of othwr XUL extensions, but having performant multicore support is critical given the shift to having many cores (as single thread performance improvements become uncommon).
XUL is still used by the browser itself.