[Certhas]

And they implemented it as strictly opt in (they ask before installing it), and fully user controlled:

"Firefox downloads and enables the Google Widevine CDM on demand, with user permission, to give users a smooth experience on sites that require DRM. The CDM runs in a separate container called a sandbox and you will be notified when a CDM is in use. You can also disable a CDM and opt out of future updates by following the steps below. Once you disable a CDM, however, sites using this type of DRM may not operate properly."