Digital Rights Management or DRM Content in Firefox is used to control the playing of DRM media like Netflix on Firefox. If this DRM feature is disabled or outdated, then Firefox may not play the Netflix content and cause the error at hand.

I was able to fix this by toggling "Play DRM-controlled content" off and on in Firefox settings. This forced the Wildvine plugin to reinstall. Toggle it off, close Firefox. Then open Firefox and toggle it on. It seemed to hang installing the plugin otherwise.

Settings - General - DRM Rights Management Content under Files and Applications