SSH port forwarding can't work with vsftpd

I have a Linux box inside some firewall which blocks the traffic to port
under 1024. To bypass this firewall, I run ssh port forwarding on it,
like this:
ssh -g -L 10000:mylinuxbox:22 user@firewall
For example, if port 10000 is not blocked by the firewall, I can access my
Linux box via ssh (port 22) remotely by setting up port forwarding as
above.

However, if I run vsftpd (not wu-ftpd, I guess that'd work) on my Linux
machine, and I again use ssh port forwarding at the firewall:
ssh -g -L 20000:mylinuxbox:21 user@firewall
I can't access my FTP server from outside the firewall even if the FTP
client is in passive mode. The situation is, I can log on the FTP server
remotely, but I can't use ls or get or put. If I do so, the FTP client
hangs up after printing out "227 Entering Passive Mode" and IP/port.

Anyway has experience with this? Maybe there's a lot of confusion here. I
can clarify the case if necessary.
Thanks in advance.

Regards,
FYJ