I installed Filezilla on a machine behind a firewall. Access to HTTP and FTP is only possible through an HTTP proxy.
I configured Filezilla to use the HTTP proxy. It fails. If I use Internet Explorer with an ftp:// URL, it works.
From an Ethereal dump, I saw that Filezilla sends CONNECT <site name>:21 to the proxy, while IE sends GET ftp://
<site name>/ . The proxy then sends the 200 OK reply to IE, while Filezilla gets a 302 Redirected (the URL to which the request is redirected, browsed with IE, says that proxy blocked access).
So, maybe Filezilla should use a plain GET, or allow the user to choose between GET and CONNECT.