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 

Replace Debian_11 with your distribution, e.g. Debian_12. Therefore have a look at download.opensuse.org/repositories

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 waterfox-g-kde

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