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.