Interesting, I didn't realize they did this. I guess they've been doing it since EME support landed[0] in 2015, but it sounds like they opted to not really publicize it. I always just set media.eme.enabled to false when I configure a new profile, but I hadn't really considered that this does not prevent the DRM libraries from being automatically downloaded.

[0]: https://bugzilla.mozilla.org/show_bug.cgi?id=1144903