I'm not sure if this is on the backlog or not but for now I think you have two options here when using Azure Firewall.
Option 1. Use an IP address prefix for your outbound public IP addresses so at least then you will know the range to whitelist.
https://video2.skills-academy.com/en-us/azure/virtual-network/ip-services/public-ip-address-prefix
Option 2: Deploy a NAT gateway to the Azure Firewall subnet, this will route all outbound traffic through the NAT gateway and use its public IP address.
https://video2.skills-academy.com/en-us/azure/virtual-network/nat-gateway/nat-overview