Very simple plugin that does a good job asking for minimal permissions. The keyboard based playback control is kinda weird - and isn't fully compatible with my keyboard media keys (it plays/pauses but won't respect the track jump keys), but is still pretty nice.

Hotkeys don't work unless Firefox is the active window, so it's not useful if I'm using another program. I'm not sure if that's meant to be possible with this but the description says "lets you control background playback".

Hey Marcin!
1) Thanks for the update, I installed it and restarted my Firefox, but...it still doesn't work! It still behaves the same way..."MediaPrevTrack" and "MediaNextTrack" are still just playing/pausing the song instead of going to the previous track or next track. I am using Firefox 66.0.2 (64-bit) on Windows 10 Home (version 1803). This is the type of laptop that I am using: https://support.hp.com/us-en/product/hp-envy-15-aq100-x360-convertible-pc/12499186/document/c05270626
Let me know if I can help you with anything to get this fixed!
2) Thanks for the suggestion, I have voted for this bug on the bugzilla site!

Works very well !!!
For those who want to use keyboard media keys for control, i've done a little AutoHotkey script:

;next song
Media_Next::
IfWinExist, ahk_class MozillaWindowClass
WinActivate
Send !+0
return

;previous song
Media_Prev::
IfWinExist, ahk_class MozillaWindowClass
WinActivate
Send !+9
return

;play/pause
Media_Play_Pause::
IfWinExist, ahk_class MozillaWindowClass
WinActivate
Send !+8
return

You're welcome :)