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.
Waterfox is based on Firefox, but forked when Mozilla utterly destroyed their reputation and completely abandoned their historical userbase to become a crappy clone of Chrome. If you need your plugins or miss them, Waterfox works well.
Thunderbird has done something similar (and there’s no alternative) having broken compatibility with “Change quote and reply format” which was absolutely essential because otherwise Thunderbird does not mark a message as read on reply/forward, which is just bizarre.