Integrated NIC on Motherboard Question

I have a question in regards to my Motherboard integrated NIC and download speeds. I have the following setup:

Motherboard: Compaq Guppy-GL6E (Asus P4GV-LA)
Integrated NIC: Realtek RTL8139/810X - I believe I have the latest driver
Processor: Celeron 340(P) 2.93 Ghz
Memory: 2GB Ram

I have Cox Cable broadband internet with download speeds of 20 mb/s. I know that speed is probably under best conditions. I have been using Firefox for a long time and I did a broadband speed test on speedtest.net . Using Firefox the best I could get is 8.6 mb/s down and 1.4 mb/s up. I did the test over a period of about a week and it wouldn't go faster. I was about ready to call Cox when I decided to try IE8. I was surprised that my average result is 16mb/s down and 1.5 mb/s up. Any ideas why this is happening with a different web browser? I prefer to use Firefox but with a 50% cut in speed I will use IE8. I have the latest version of Firefox so I was thinking maybe I have a setting wrong.

Any help or suggestions would be great!

Best Regards,
AZFlying

edit: I also tried Google Chrome and Safari with the same 6-8mb/s downloads. IE8 seems to be the only browser that is showing the higher download speeds.