Slow downloads

I just got a fiber optic internet connection than downloads at 25000 kbps and uploads at 3500 kbps. I am hooked up to an Ethernet line from the modem. I am only getting around 9000 kbps download readings but full upload. However an old PC I hooked up the same way is getting the full download. My provider said it must be a configuration issue. I've tried both safari and Firefox, same results. Any suggestions?
Mac Pro
Dual Quad core 2.8ghz
10 gig ram
OSX 10.5.8

Moved to appropriate forum

Ethernet settings are few. Open your System Preferences, Network, and click the Ethernet selection on the left. Make sure you're using DHCP and that the correct location is selected in the "Location" box.

After that, check the modem and router settings. You mentioned you have a fiber connection. It would help if you told us who your ISP is. I also use a fiber connection and my ISP is Verizon. Fiber connections usually use a specialized router or modem and some fiber connections like mine incorporate the modem and router into one unit.

Regards.