To be more clear, what a firewall is going to want more of is:
fast NICs (especially ones that don't need CPU power to run) - I like Intel-powered ones
not necessarily in that order. All an SSD will do is make it boot faster. You can tell if you need more CPU by running top while the firewall is "slow" and seeing if the load average is well in excess of 1.
Assuming the firewall is your problem to begin with.
i5-2500K@4.3|Asus P8P67-LE|8GB DDR3-1600|Powercolor R7850 2G|SanDisk Ultra II 480GB|1988 Model M|Saitek X-45|Logitech MX 518 & F310|Dell 2209WA|Sennheiser PC151|Asus Xonar DX