Make sure you have the latest Widevine version (4.10.2391.0). You can toggle DRM off/on to see if th

Make sure you have the latest Widevine version (4.10.2391.0).

You can toggle DRM off/on to see if that makes Firefox (re)load DRM components.

Settings -> General: Play DRM

https://support.mozilla.org/en-US/kb/enable-drm

You can possibly check the XHR requests in the Browser Console if this doesn't work.

https://developer.mozilla.org/en-US/docs/Tools/Browser_Console