I also noticed that when I play the same song side by side using the macOS desktop app vs firefox webplayer that the volume from the desktop app is lower than the firefox webplayer. Both are on full volume and the desktop app is still lower volume and sounds more "dull" like mids and bass are dampened. Firefox sounds as supposed to. Played around with the app settings but couldnt find anything related to volume output or equalizer. Anyone else had this happen and does this also happen with Chrome or other browsers?
@SebastVAIO, we know it's the browser (Safari), not the OS. As I stated in the edit to my original post, I tried changing the user agent to Chrome and allowing the Widevine plugin, but it still did not work.
Unfortunately IE 11 is also no longer supported. It seems they are really trying to push installing the webplayer, apparently not caring at all that a lot of their user base cannot install third party software.
Also Firefox ESR v.45.8 (extended support release) is no longer supported.
This must be a joke or a massive bug... spotify will loose me as a customer if they don't fix this. Firefox ESR and IE11 are the only browsers i can use at work. If i cannot use the webplayer, there is no need for me to pay any monthly subscription.