This is a WebExtension that indicates whewn you're in a non private GitHub repository.
Pick the latest release and click on the *.xpi asset. Firefox, or any other browser that supports WebExtensions, should then ask you if you want to install the extension.
Use web-ext for automatic loading and re-loading of the extension. I recommend direnv and nvm for the local development setup. Afterwards you can just run
$ npm installto install all dependencies.