If you have a requirement to reboot both members in a firewall cluster, then you will need to reboot the inactive unit first, as it does not reboot if you reboot the master. To complete this, you can connect to it via the HA interface from the active Firewall.
ActiveFW # execute ha manage 0 admin
Warning: Permanently added '169.254.0.1' (ED25519) to the list of known hosts.
RedundantFW # execute reboot
This operation will reboot the system !
Do you want to continue? (y/n)y
System is rebooting...
Once this has been completed, you can then reboot the active firewall
ActiveFW # execute reboot
This operation will reboot the system !
Do you want to continue? (y/n)y
System is rebooting...