Did you use this instruction? If so, i think you forgot this step
wget -O- https://download.opensuse.org/repositories/home:/bgstack15:/aftermozilla/Debian_Unstable/Release.key | sudo gpg --dearmor -o /etc/apt/keyrings/home_bgstack15_aftermozilla.gpg
Did you use this instruction? If so, i think you forgot this step
wget -O- https://download.opensuse.org/repositories/home:/bgstack15:/aftermozilla/Debian_Unstable/Release.key | sudo gpg --dearmor -o /etc/apt/keyrings/home_bgstack15_aftermozilla.gpg