I personally don't belive a firewall would have anything to do with it.
After all a firewall only controls (blocks) incoming or out going info from you computer, frame rates are not information that is being transfered from your computer or another or vice versa. The only thing I can see a firewall doing is using system resources but not enough to effect framerates.
But it is true that the least amount of "back ground" programs running will boost frame rates sometimes dramatically, so if your not connected to the Internet than yes do turn it off, the biggest hit in frame rates from back ground programs I have experianced is from anti-virus software, which also should be disabled if not connected to the internet.
I don't use cable so I will have to belive those who do about even if the internet is disconnected that you are always "live" so I would also recommend thier advice as to leaving the firewall and anti-virus on!
This is the reason I have never got to experiance and enjoy multi-player flying, my computer isn't strong enough to fly smoothly with my anti-virus and frewall on so I solo
