Also, AFAIK notification only pertains to one part of DMCA: the "safe harbor" for infringing user-posted content. DRM-breaking tech is banned under a separate s

Also, AFAIK notification only pertains to one part of DMCA: the "safe harbor" for infringing user-posted content. DRM-breaking tech is banned under a separate section that, to my knowledge, has no equivalent protection for the "service provider". So if Mozilla's lawyers became aware of this add-on, they might have reasonably concluded that hosting it was an unacceptable risk.