It's a nice side-effect to be compatible with Chrome-extensions, so that even if Chrome continues eating up the browser marketshare that there will still be ext

It's a nice side-effect to be compatible with Chrome-extensions, so that even if Chrome continues eating up the browser marketshare that there will still be extensions available on Firefox, but the main-motivation was simply that Firefox needed a proper extension API and then why design an API from scratch, if there already is one that does everything you need.