1. download uBlock0_1.16.4.24.firefox-legacy.xpi

wget -c https://github.com/gorhill/uBlock-for-firefox-legacy/releases/download/firefox-legacy-1.16.4.24/uBlock0_1.16.4.24.firefox-legacy.xpi

2. open uBlock0_1.16.4.24.firefox-legacy.xpi using Seamonkey