It seems like Steam is always getting hit by DOS attacks. I hope that in the future, firewalls will get good enough to prevent something like this from happening.
Even IF firewalls become able to magically drop SYN packets only from a DDOS attacker (or even a full page request, depending on how they adjust their attack to counter your magic firewall), the idea of a DDOS is to utilize all available sockets AND/OR flood an internet connection to the point of denying legitimate traffic. Even if your firewall is properly dropping packets (as they currently DO have SYN flood, etc type filters btw), it does nothing to improve your upstream bandwidth utilization. If you have a 10Gbps trunk to the internet, but are being hammered with 30Gbps of DDOS traffic, you gain nothing. It is dependent on your upstream provider assisting in stemming the tide. Once that flood is at your door (firewall), you're mostly already sunk.