I can get Spotify to work in Vivaldi 1.11.917.17-1. However using the exact same Widevine shared object (libwidevinecdm.so) in Chromium 59.0.3071.109 (Ubuntu 16.04), I get the familiar error message, "Playback of protected content is not enabled". Vivaldi, like Chromium, uses Blink.
You are using an unsupported system configuration. Firefox 52.x is relative new supporting DRM.
I'm not too sure what you are talking about, Firefox is on the supported list of browsers...
A little of history from wikipedia, Firefox started supporting DRM via NPAPI since version 47 until version 52, when decide to remove NPAPI support (March 7, 2017). Prior to this date, Firefox has been steadily improving their platform adding features to become native Web APIs, mirroring actions of other browsers. From then, as you can see here: https://wiki.mozilla.org/QA/Widevine_CDM, Linux was and still is NOT be part of the test scope, so it will be an slow progression.
So this is not an issue of Spotify, probably the message page could be more specific, but there is nothing else to do (in some flavors of Linux).