Is anyone using NMS to monitor Watchguard firewalls? Can you tell us what probes you are using? Is SNMP available for query and/or traps?
Though there may be nothing out of the box you should be able to roll your own fairly easily.
Have a look at SNMP, both gets to look at things like mem/cpu/temp etc, and traps for the firewall to send you alerts.
The firewall can also emit syslog, so the syslog gateway will pick up these and can be used to raise alerts.