Although there are no official Debian repositories for Waterfox, it can be installed from a repsoitory with APT.

Repository: opensuse.org

Therefore use this commands:

sudo apt update && sudo apt upgrade -y
sudo apt install software-properties-common apt-transport-https curl -y
wget https://download.opensuse.org/repositories/home:hawkeye116477:waterfox/Debian_11/Release.key 
gpg --dearmor Release.key && cat Release.key.gpg | sudo tee /etc/apt/trusted.gpg.d/home_hawkeye116477_waterfox.gpg > /dev/null 
rm Release.key*
echo 'deb http://download.opensuse.org/repositories/home:/hawkeye116477:/waterfox/Debian_11/ /' | sudo tee /etc/apt/sources.list.d/home:hawkeye116477:waterfox.list
sudo apt update 
sudo apt install fonts-lyx ffmpeg -y
sudo apt install waterfox-g-kde

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