I had the same problem and here's how I fixed it. I use Internet Explorer, but the directions should be close for others. Here are the steps in order.
Download the latest firmware from the linsys website.
Access you router at http://192.168.1.1. If the user name and password screen appears, the default is admin for username and admin for password, at least it was for mine.
Upgrade the frimware. If you need help upgrading firmware, liksys has a tutorial on the site.
Click on Setup.
Under the Basic Setup screen, change the Local IP address to 192.168.2.1. (This is straight from linksys)
Make sure that the router and modem are hooked up correctly.
Go to Internet Options under Tools.
Click on the Connections tab.
Click on Lan Settings.
Check the box next to "Automaticaly detect settings".
You should be able to access the internet.
This is what worked for me. Hope it helps.
Feel free to email me with questions.