A lot of the guides for resetting the root password on an OPNSense box assume a UFS root partition. The password recovery steps do not work if you installed OPNSense with a ZFS root partition. If you try to follow the steps you get a lovely error about “unrecognized filesystem”

The process for ZFS (thanks to this article) is instead to run the following commands:

zfs set readonly=off zroot
zfs mount -a

Once that is done, you can proceed with the rest of the steps. Password recovery steps in full:

1. Press the number 2 immediately on boot to go into single user mode
2. Press enter when prompted for shell
3. Make ZFS read/write:
   1. zfs set readonly=off zroot
   2. zfs mount -a
4. Reset password
   1. opnsense-shell password
5. Reboot