If settings load on Firewall tab, but you get "The firewall settings could not be updated." error message when trying to save settings there is a file permission issue on your server.
Make sure that webserver user has read/write permissions to vendor/webarx/firewall directory recursively.
To give ownership over firewall folder to www-data user and www-data group , use following command over SSH. Replace path to firewall package if you don't use composer for package management in your project.
chown -R www-data:www-data vendor/webarx/firewall