Synchronizing Firewall Configuration with NSM
The management status of a firewall changes to Unmanaged state when the firewall is locally modified. You need to Synchronize Firewall configuration with NSM to set the device in Managed state.
To synchronize firewall configuration with NSM:
Navigate to Manager View | Firewalls > Inventory. Click the Ellipses icon in the Action column for the firewall you want to synchronize the changes with NSM, and select Synchronize Firewall.
 
In the Synchronize Firewall dialog, click OK.Â

No need to review differences, as NSM will discard whatever it has and retrieve current running configuration from firewall.Â