What is the best method to downgrade the version of Fireware OS on your Firebox...

Restore a saved backup image that was created for the device before the last Fireware OS upgrade.

Use the Upgrade OS feature in Fireware Web UI to install the sysa_dl file for an order version of Fireware OS.

Change the OS compatibility setting in Policy Manager to downgrade the device. Then use Policy Manager to save the configuration to the device.

Use the downgrade feature on Policy Manager to select a previous of Fireware OS.