The browser plugin is written in Typescript. To build it, make sure you have nodejs / yarn installed.
Then:
yarn install
yarn webpack
yarn build
This will create a zip in web-ext-artifacts
that can be uploaded to addons.mozilla.org
yarn install
yarn dev # Debugs the plugin in Firefox via "web-ext run" and runs a backend service in Ruby + Sinatra
yarn dev-frontend # Debugs the plugin in Firefox via "web-ext run" without the backend service
yarn backend # Runs the backend service in Ruby + Sinatra