SSH-tunnels don't work even when enabling "Allow TCP forwarding" in the Web-UI. The default option "PermitOpen none" in /etc/ssh/sshd_config prevent SSH-Tunnels to work. After deleting "PermitOpen none" and reload the OpenSSH-Server (/etc/init.d/sshd reload) the SSH-tunnels work as expected.
https://patchwork.ipfire.org/patch/3737/
https://git.ipfire.org/?p=ipfire-2.x.git;a=commit;h=fe0984e07bb48922acae5a4993503455df9dec0d
https://blog.ipfire.org/post/ipfire-2-25-core-update-154-available-for-testing