I eventually received confirmation from the SSH server owner that indeed port forwarding was disabled in their sshd_config file (AllowTcpForwarding: “no”).
At my request, they reset AllowTcpForwarding to “yes”. That solved the connectivity problem. Now I can connect Firefox to Internet using Tunnelier’s SOCKS proxy and my host’s SSH server. This means secure and anonymous browsing.
还有一种说法，是大网关的临时性 block 指令强行切断了连接，过一会儿就恢复。