The standard ftp command/client does not support IPv6 resolution.

A very powerful IPv6 ftp client alternative is lftp.

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id@acer:~$ lftp ipv6.idrobot.net
lftp ipv6.idrobot.net:~> dir
-rw-r--r--   1 ftp      ftp             0 Aug 26 17:28 test.txt

And of course Firefox does FTP over IPv6:
ftp://ipv6.idrobot.net OR ftp://[2a01:4f8:131:51e2::10]