Generally, considered to be an advanced firewall, CSF blocks external IPs (Inbound Connections) to avoid Brute force attack or attack similar to that (Multiple login failures, hacking attempts, etc.) on a server. The firewall might block the connection while uploading files using Filezilla. It might be possible that Filezilla is trying to open multiple simultaneous connections to speed up the transfer. The firewall may interpret it as brute force or port scan activity and block the IP address from where the connection is originating.