If you set up the cluster in peer-to-peer mode I think you will find that the changes made to the second appliance will re-synchronize to the 'primary'.
However, if you did configure the cluster as master/slave (primary/secondary - I can't remember which name it uses) when you bring the primary back into service yoy may well find that any changes made to the secondary will be lost the next time the secondary re-synchronizes with the primary.
I have to confess I am unsure as I have been configuring clusters in peer-to-peer mode for so long now I can't remember exactly how the older master/slave configuration actually worked.
Thank you for your reply. My cluster is set as primary/standby. There were alot of rules added to the secondary firewall. Is there any way to reinstate my original primary firewall to the cluster without losing the firewall rules added to the secondary firewall?
if you change old secondary to new primary, then the old primary will be the secondary. then your old primary will get the rules from new primary (old secondary) or your cluster have to remove first, then create a cluster again with your secondary will be the Primary and your old primary will be secondary.
If you're at 8.3.2 and later then the primary will not overwrite the changes made on the secondary while the primary was offline.