Can you please give us a bit more information. I suspect from what you are saying that you do have a wireless connection to the internet, but that its speed varies. Is that correct?
If so, do not be too alarmed - that is normal. The speed will change with the distance between the two ends of the wireless link, what lies between them and with interference from other devices. It is almost certainly not the firewall, or you would not be able to surf at all.
The same frequency band used by wireless networking is also used by telephones, bluetooth, tv transmitters (room to room) and, sometimes, leaky microwave ovens. You can also get interference from other users of wireless networks in your neighbourhood.
One of the first things you can try is change the channel of your wireless network. Change the number by 5, ie if you are on channel 1, change to channel 6 and v.v. remember to change at both ends of the network, router and pc.