Firewalls can't do anything to prevent DDoS, you'll still consume bandwidth from your ISP your firewall can just drop the traffic but you'll still be billed for the traffic hitting your firewall... often the traffic will far exceed your port speed under a true DDoS attack.
The only real way to mitigate DDoS is to use something like CloudFlare or Prolexic from Akamai, which have multiple terabits of bandwidth available spread across the globe and use some fancy AnyCast stuff to split the load across DCs for the same IP. Expect to pay $5-10k per month USD, minimum, for such services.
Basically, you need to educate yourself more on what DDoS really is, you can't buy hardware or software to prevent it.