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.

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

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?

I'm not sure what decoupling means, and whether it's of any technical significance beyond bragging rights, but chromium won't support drm out of the box either because widevine is obviously not a part of chromium. You can take the widevine library from chrome and make it work with chromium if you jump through some hoops.

Not sure how it works on other OSs, but in my experience on Linux, Chromium is installed without Widevine DRM, Flash, or any proprietary stuff, and if you want that you have to install it separately.