A firewall will most likely categorise our scanner as a bot and will block us. A hacker can often find and abuse vulnerabilities manually. A firewall can help prevent some attacks (automatic and manual) but certainly not all. There are plenty of vulnerabilities that can be exploited within the intervention of a firewall.
Whitelisting our IP-range can be done temporarily for the duration of the scan. You can always contact us to agree on a specific date/start time by sending an email to support@forus-p.com.