I tried the PreMid extension and PreWrap but neither actually worked for me, but this did

Works pretty correctly on Windows 10 64bits, Firefox 75.0 (4 bits) and Discord regular Desktop app.
Only if you don't forget to install both of dependencies :

https://addons.mozilla.org/fr/firefox/addon/discord-rich-presence/
&
https://github.com/lolamtisch/Discord-RPC-Extension/releases/latest

The problem is that, once you change tab, even if the song is still currently playing. Plugin will stop displaying status on discord.
At least for me, if i go on another tab it stops.
While tab is active, it displays correctly though