Normal Firefox builds also provide a checkbox under the Content settings to disable it. As far as I recall, I've never touched the setting, but it's disabled here.

I suppose EME-free builds prevent even the possibility of enabling it.

You can technically re-enable EME in the "EME-free" Firefox builds by setting the "media.eme.enabled" pref in about:config. The term "EME-free" is a bit of misnomer. The "EME-free" builds have two differences from regular Firefox builds: the Widevine CDM downloader is disabled and the "Play DRM content" setting checkbox is hidden.

I see, thanks. I hoped they actually disabled EME functionality at the build time. That would have made more sense.

Well, Firefox does not contain EME functionality itself – if it’s needed, and activated, it just downloads the binary and runs it.

The EME-free builds just have that download code disabled, and the setting for it hidden.

It supports usage of EME related API which sits around the blob. Having no such support altogether (cut out at build time), would make the blob irrelevant to begin with.