Preinstall uBlock Origin for Firefox system-wide or bundle with .app

Unfortunately there is no port anymore:
https://www.freshports.org/www/xpi-uBlock_origin/

So, something along these lines?

```
sudo sh
URL=$(wget -c "https://api.github.com/repos/gorhill/uBlock/releases" -O - | grep browser_download_url | grep 'firefox.signed.xpi"' | head -n 1 | cut -d '"' -f 4)\n
wget -c "$URL"
sudo mkdir -p /usr/local/bin/browser/extensions
sudo mv ./*firefox.signed.xpi "/usr/local/lib/firefox/browser/extensions/uBlock0@raymondhill.net.xpi"
```