Mozilla has implemented DRM support in Firefox. I think Mozilla is more concerned with reducing dependence on nonfree software than supporting free software.

Just to clarify, I don't fault Mozilla for having that attitude, but that doesn't mean I prefer it. I would prefer that they prioritize FOSS platforms like Linux and \*BSD, as well as getting Firefox on platforms like Plasma Mobile, PostmarketOS, and other GNU/Linux-based mobile operating systems. Developer adoption is a necessary (but probably not sufficient) precondition for general adoption, which is why developing for these platforms is important even though they are new/unpopular.