I checked Youtube and everywhere else and they play fine.
Only Soundcloud has this problem, all music is only coming through one speaker, making it unusable under Firefox.
Best answer by SevenKnights
It's not Firefox, it's not Chrome. It's Pulseaudio.
In /etc/pulse/daemon.conf set:
enable-lfe-remixing = yes
pulseaudio -k && pulseaudio --start
Soundcloud and some old youtube songs will work again after this.