What are the steps that use to configure to backing up configurations of WatchGuard Firebox firewalls using NCM??
Have you tried this template: https://thwack.solarwinds.com/content-exchange/network-configuration-manager/m/device-templates/394 ?Essentially there are two tasks you will need to sort.
Task 1: Configure a connection profile - this is the USER ID, password and protocols for logging on and downloading the config (FTP, SSH, etc).
Task 2: Use or configure up a Device Template (which is the item I've linked above).
After completing Task 1, hit the test button to make sure you can talk to the device. If the test is successful, but you can't d/l a config then the issue is with your device template, and you'll need to edit that to reflect appropriate commands for accessing the device.