Greasy Fork

It works

But I want to work focusing any app in desktop and custom keys

focusing any app

This is currently not possible with Firefox. But I have written a AutoHotKey-Script that sends the keys to Firefox if it's not focused in the moment.

custom keys

These could be changed directly in the Userscript, or you could specify the custom keys in the AHK-Script.

If you need an exe of that script pm me, otherwise, here is the code of the AHK-Script:

Media_Play_Pause::ControlSend,ahk_parent, {Media_Play_Pause}, ahk_exe firefox.exe
Media_Next::ControlSend,ahk_parent, {Media_Next}, ahk_exe firefox.exe
Media_Prev::ControlSend,ahk_parent, {Media_Prev}, ahk_exe firefox.exe
$^!p::ControlSend,ahk_parent, {Media_Play_Pause}, ahk_exe firefox.exe
$^!.::ControlSend,ahk_parent, {Media_Next}, ahk_exe firefox.exe
$^!,::ControlSend,ahk_parent, {Media_Prev}, ahk_exe firefox.exe