I updated Firefox (to version 64.0) today and got an orange message bar on open.spotify.com stating that "firefox is installing components needed to play the audio or video on this page". This "installation" never succeeds.
After a lot of Googling, I determined that some WideVine plugin was the problem. DRM was enabled, WideVIne was enabled, but the problem persisted.
I finally resolved it and figured I would post it here because I didn't find the solution anywhere else. WideVine's auto-update setting was disabled by default in my browser. To fix:
1) Go to about:config
2) Search for the "media.gmp-widevinecdm.autoupdate" setting.
3) If the setting is false, double click to put it to true.
4) Reload the open.spotify.com page and the orange message should go away!