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
Review the configuration differences between NSM configuration and the local firewall configuration.
Review the pending commits.
Click OK in the dialog.
Synchronization process runs.
The firewall is now managed by NSM, thus the CONFIGURATION status changes to Managed in the Firewall Inventory page.