Here's a couple of things to try.

First, please post what version of knoppix and your hardware setup including your network equipment.

Of course you've checked the connection settings in edit, preferences in firefox.

If you want to monitor the network traffic you could try using ethereal:

First you need to set a root password: k-->knoppix-->rootshell
type: passwd , enter a password then minimize the window

Then start ethereal, k-->internet ethereal. Enter the password you just set.

Click capture, options and check the box for Update packets in real time , and then click the box for Automatic scrolling of live capture, then click the start button.

Now start firefox . Take a look at the ethereal output and see if anything suggests what the problem might be. you can stop the capture and scroll up to look over things.

Theres also another browser you can try: Lynx, which is not graphical but it might be interesting to see if it works. k-->internet--lynx.

HTH