Я использовал эти команды для установки:
sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/hawkeye116477:/waterfox/xUbuntu_Next/ /' > /etc/apt/sources.list.d/home:hawkeye116477:waterfox.list"
wget -nv https://download.opensuse.org/repositories/home:hawkeye116477:waterfox/xUbuntu_Next/Release.key -O Release.key
sudo apt-key add - < Release.key
sudo apt-get update
sudo apt-get install waterfox
Вот какую ошибку я получаю:
W: GPG error: http://download.opensuse.org/repositories/home:/hawkeye116477:/waterfox/xUbuntu_Next InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5E62D791625A271E
E: The repository 'http://download.opensuse.org/repositories/home:/hawkeye116477:/waterfox/xUbuntu_Next InRelease' is not signed.