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

Hello, I understand that it would be very useful to implement it, but there are some extensions that do it specifically for Soundcloud, so at the moment there are no plans to add support for more platforms.