But that's the entire point of using global shortcuts - I never have my media player focussed. I specifically had to enable global media shortcuts in it and Firefox should do the same: ask the user if they want this feature, and not assume that no other application might want to get hold of a global resource. As always, the mind game "what happened everyone did it like that?" is very important here.
Anyway, I just got the Firefox update and it seems like my media player seems to "win" over the global shortcut from Firefox that I didn't ask for. So all is good for now...
At least on KDE media focus is first come first serve. The first media player to start playing gets the media keys until it stops and then whatever else is playing or plays next gets them.
The only unintuitive part is that maybe it should first try to play / pause the active window before delegating to that priority.
Anyway, I just got the Firefox update and it seems like my media player seems to "win" over the global shortcut from Firefox that I didn't ask for. So all is good for now...