On the Nokia 7 plus devices we have, there is still no Widevine DRM support.
An exception is thrown when we try to access a MediaDrm instance. Using the 'DrmInfo' app from the Play store also shows this.
We need Widevine for our app (and I assume others do to).
Hopefully this gets resolved before official release !
Can't comment any special knowledge there. I do know for example on Debian that there exists a file called /etc/firefox/firefox.js which overrides user set settings can have some things set in it which prevent Widevine from downloading properly. (Or did in past versions of the Debian Firefox package.)