Expose Metadata to firefox's media-control feature

Since version 81 Firefox supports controlling media (https://support.mozilla.org/en-US/kb/control-audio-or-video-playback-your-keyboard?redirectslug=firefox-media-controls&redirectlocale=en-US#firefox:linux:fx82).

Under Linux this exposes the media to mpris.

Playing around with playerctl, this works so far with funkwhale. But the metadata only expose the tab-title (which leads me to guess it doesn't expose them at all and firefox just defaults to using the tab-title).

As an Example how it looks with funkwhale:

$ playerctl metadata
firefox mpris:trackid             '/org/mpris/MediaPlayer2/firefox'                                                      
firefox xesam:title               ♫ Lament for Boromir – Clamavi De Profundis ♫ – Subscribed Channels – EorlBruders Music
firefox xesam:album                                                                                                      
firefox xesam:artist                                                                                                     

And here an example how it looks with youtube:

$ playerctl metadata
firefox mpris:trackid             '/org/mpris/MediaPlayer2/firefox'                               
firefox xesam:title               Chamberpoint Ep.8: Fire and Water                               
firefox xesam:album                                                                               
firefox xesam:artist              Kruggsmash                                                      
firefox mpris:artUrl              file:///home/david/.mozilla/firefox/firefox-mpris/1622971_3.png 

Since we do have all that metadata in funkwhale, it would be nice if they could also be exposed to firefox's media-control, in particular for scripting or usage in widgets etc.