That design decision makes sense, to decouple the EME module, and great that Mozilla offers a prebuilt binary without it, as an opt-out. I wonder if Chromium (or a fork) can also be built without this feature?

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

The normal (not the "EME-free" ones) builds download the Widevine CDM from Google shortly after being run. The EME-free builds have a boolean pref pre-flipped, so that it doesn't download the Widevine CDM unless you manually flip the pref the other way. If you download a non-"EME-free" build and flip the pref, Firefox deletes the Widevine CDM if it already downloaded it.

In summary, if you already have Firefox, you don't need to go download a separate build to opt out of DRM. You can just uncheck the "Play DRM-controlled content" checkbox in the prefs to get to the same configuration.

> I'm not sure "doesn't work with most major video sites" is an actual selling point.

You just have to reframe it to something like: "protects you against websites that try to plant DRM-malware on your computer".