FileZilla does not use own ports

[zarggg]

When retrieving data from a remote host, the data is
sent to a local port that is not explicitly opened by
FileZilla.

The result is that a firewall that unsolicited (or
unexpected) inbound packets will reject all inbound
data while attempting to use this program in a normal
manner.

Example: FileZilla opens port 3825 on the local
machine, but the data is being sent to local port 5004,
which is not open by any program on the local host. As
a result, the firewall on the local host rejects the data.

[Tim Kosse]

There must be a problem with some (maybe your) firewall or a
router between the server and the client.
FileZilla always opens the port before sending the address
to the server.

Lots of misbehaving routers and firewalls tend to inspect
ftp connections and modify the PORT and PASV commands and
their replies.