I’m sorry to hear about your bad experience.
Right now, Firefox doesn't support global keyboard shortcut so there is no official way to do it. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1411795
I created a workaround using AutoHotkey program, but I can't be sure it'll work. See: https://github.com/jerosa/spotify-hotkeys-firefox/issues/11