The build requires python 3.7 and node 8+. Before running build, you must prepare the build environment once by running
$ pip3 install -r requirements.txt
$ npm install
After this, build.py rebuilds the XPI. Without parameters it just grabs the sources,
creates the install.rdf and zips it up. If you pass --release, it will
also update update.rdf
The build (ab)uses node solely because npm has good built-in facilities
for version management. npm version manages versions nicely and also
checks you're not bumping versions while your working dir is dirty. It
also stores the version number in package.json, where build.py picks
it up.
npm version will automatically run build.py after bumping.