firewalld: Services become inaccessible due to an error

I found that all web-based services became inaccessible on my FreedomBox running Debian testing due to an error in firewalld. SSH access was still working, so I was able to find this error in the systemd service logs.

Oct 25 13:38:15 freedombox firewalld[650]: ERROR: '/usr/sbin/ip6tables-restore -w -n' failed: Warning: Extension MARK revision 0 not supported, missing kernel module?
                                           ip6tables-restore v1.8.10 (nf_tables): MARK target: kernel too old for --or-mark
                                           Error occurred at line: 2
                                           Try `ip6tables-restore -h' or 'ip6tables-restore --help' for more information.
Oct 25 13:38:15 freedombox firewalld[650]: ERROR: Failed to load user configuration. Falling back to full stock configuration.
Oct 25 13:38:15 freedombox firewalld[650]: ERROR: COMMAND_FAILED: Direct: '/usr/sbin/ip6tables-restore -w -n' failed: Warning: Extension MARK revision 0 not supported, missing kernel module?
                                           ip6tables-restore v1.8.10 (nf_tables): MARK target: kernel too old for --or-mark
                                           Error occurred at line: 2
                                           Try `ip6tables-restore -h' or 'ip6tables-restore --help' for more information.