Instalation of Firefox on Bullseye or Bookworm is easy with Debian Sid repository.
Process is very simple and have only 3 steps.
1. Step - Add Debian Sid repository:
echo "deb http://deb.debian.org/debian/ unstable main non-free contrib" > /etc/apt/sources.list.d/sid.list
2. Step - Pin packages from Sid (prevent upgrade all to Sid):
nano /etc/apt/preferences.d/99-debian-sid
Package: *
Pin: release a=unstable
Pin-Priority: 10
3. Step - update repository and install Firefox
apt update
apt install firefox
Enjoy in latest Firefox !!!
