Story

Firefox does not load this module directly. Instead, we wrap it into an open-source sandbox . In our implementation, the CDM will have no access to the user's hard drive or the network. Instead, the sandbox will provide the CDM only with communication mechanism with Firefox for receiving encrypted data and for displaying the results

So they are adding a way to load a module that supports DRM playback. They are not adding DRM to Firefox. They are adding DRM capabilities to Firefox. I think that's an important distinction.