I guess you cannot use the port 21, it is strictly reserved for FTP. The recommended web ports are 80, 8080 or any above 1024 (the first 1024 ports are reserved for special purposes, but all above 1024 is free to use).
Firefox thinks HFS is malware, as it attempts (well, actually you) to open a HTTP connection on a port reserved for FTP (FTP is a completely different protocol).
Your solution is: Don't use port 21, but 80, 8080, or >1024.