Recovering the administrator password is not possible–the password must be reset using the following procedure
Boot the firewall into maintenance mode and load a previously saved named config.
To boot the firewall into maintenance mode, connect to the console via the console port and a terminal software.
Reboot the firewall and keep pressing ‘m’ (or ‘maint’ for newer versions).
Choose a previous version of the running config for which the administrator password is known and reboot the device with this config.
If a previous config cannot be loaded or the password is unknown for all saved configs, the firewall will need to be factory reset in order to configure a new administrator password.