"ucblock.xpi" was created as follows:

git clone https://github.com/UCyborg/uBlock-for-firefox-legacy.git
git clone https://github.com/uBlockOrigin/uAssets.git
cd uBlock-for-firefox-legacy
git checkout master
tools/make-firefox-legacy.sh all
ls -l uBlock-for-firefox-legacy/dist/build/uBlock0.firefox-legacy.xpi

# then move and rename the ".xpi" file produced.