The Waterfox PPA changed recently. The following let me update from 56.2.8 to 56.2.10 (between which the old PPA was removed).
First remove the old hawkeye PPA from your sources list, then:
echo "deb http://download.opensuse.org/repositories/home:/hawkeye116477:/waterfox/xUbuntu_18.04/ /" | sudo tee -a /etc/apt/sources.list wget -nv https://download.opensuse.org/repositories/home:hawkeye116477:waterfox/xUbuntu_18.04/Release.key -O Release.key sudo apt-key add - < Release.key sudo apt update sudo apt upgrade
Note Ubuntu 18.04 = Mint 19/19.1 the 18.10 deb fails.