Do your firewalls really require an explicit exception for outgoing (!) TCP on port 80? I would say that it prohibits a whole host of useful web technologies.
However, it is not possible to know the exact IP address or subnet on which the public high-profile website runs. There may be DNS-based load balancing, local mirroring (a la Akamai), simple old server migrations. Trying to push the IP address down and expecting it to remain that way is completely useless. DNS resolution exists for some reason; Google provides services based on the site name, not the IP address. An IP address is an implementation detail that cannot be relied upon positively.
If you are not relaxing the firewall, consider accessing Google through a proxy server outside the firewall.