Sharepoint can "do" that but it's a matter of how the firewalls are set up rather than Sharepoint. To my knowledge, Sharepoint doesn't have any specific tools to deal with going through firewalls. Depending on the degree of security you want, you have to configure the firewall to let users access Sharepoint. So it's really a firewall question.
I use Sharepoint behind a firewall and users outside of the firewall must VPN in to access it.
dino