A factory default FireBrick configuration will work with a ZTE MF823 dongle without any additional configuration

The FireBrick will DHCP an address in the 192.168.0.x range from the ZTE dongle
You can access the dongle's web interface via http:192.168.0.1
There will be NAT performed by the FireBrick and then again by the dongle

Changing the ZTE MF823 to 'modem' mode

Warning: This is still in progress and does not work

The ZTE823 works as an Ethernet device by default, it is possible to change the mode so that it works as a modem. The idea here is that rather than the dongle NATing the connection it can be a modem and the FireBrick will be given the public WAN IP address directly.

Switching to Modem Mode

Disabling Autoconnect

Soe of this information came from: archlinux.org

Disable auto-connect:
- Go to http://192.168.0.1
- Click on Home
- Click Disconnect
- Click Settings
- Click Network settings
- Click Dial-up settings
- Click the radio button labeled 'Manual'
Put the Dongle into download mode:
- In your browser, go to: http://192.168.0.1/goform/goform_process?goformId=MODE_SWITCH&switchCmd=FACTORY
Next, the FireBrick should see the dongle as a modem
TODO - HOW TO CONFIGURE THE FIREBRICK TO USE THE DONGLE

Switching back to Ethernet mode

To put the dongle back to Ethernet mode you will need to connect the dongle to a computer in order to send it some commands via a serial terminal

gtkterm

Open gtkserial or another serial terminal (or simply echo the following AT commands to /dev/ttyUSB2) (or maybe another dev/ttyUSBx):

ATZ
AT&F
AT+ZCDRUN=9
AT+ZCDRUN=F

Disconnect the modem from the computer and reconnect it to the FireBrick

What mode is it in?

On a linux system, if you run lsusb you'll see something like:

Bus 002 Device 030: ID 19d2:0016 ZTE WCDMA Technologies MSM

The 0016 is the mode:

1225 – Default Mode. Available USB Mass Storage Device with CD-ROM and card reader. Corresponds to AT+ZCDRUN=9+AT+ZCDRUN=F
1403 – Operating Mode. Available RNDIS adapter and Mass Storage Device. Corresponds to AT+ZCDRUN=8+AT+ZCDRUN=F
1405 – CDC Ethernet Mode (the one we need). A mode similar to that described above (1403). Included in Linux after starting usb_modeswitch c default settings.
0016 – Download Mode. Under the name of ZTE., but simply a mode where available diagnostic port and two command (analog modem port and PC UI devices Huawei). Corresponds to AT+ZCDRUN=E
0076 – "real" Download Mode. Includes a standard for devices running QC methods.