Thanks for your kind words! At the moment there are two icons, one for the light theme (green and white) and another one for the dark theme (black and white). You can see them in the github repo: https://github.com/jerosa/spotify-hotkeys-firefox/tree/master/icons

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

Thanks to you for using it ^^

Thanks for your kind words!

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.

Thank you for your words!
I can add this features. Please go to https://github.com/jerosa/spotify-hotkeys-firefox/issues/14 for more discussion.

Glad to know the addon is what you were looking for! Thanks for the review.