Do your firewalls really require an explicit exception for outgoing (!) TCP on port 80? I'd say that prohibits a whole slew of useful Web techniques.
That said, it's not possible to know for sure the IP address or subnet that a high profile public website is running on. There may be DNS-based load balancing, local mirroring (a la Akamai), plain old server migrations. Trying to pin the IP address down and expecting it to stay that way is utterly useless. DNS resolution exists for a reason; Google provides services based on site name, not on IP address. IP address is an implementation detail that you cannot reasonably rely on.
Unless relaxing the firewall is an option, consider accessing Google via a proxy outside of a firewall.