Mozilla Firefox

I use Mozilla Firefox and am having trouble with websites looking normal. For example, I use Craigslist as my home page. When I open Mozilla, the background on Craigslist is all white. If I open Craigslist on IE or on Safari, the categories are separated by lightly shaded boxes and the city title is in a dark gray box.
I have tried changing the color and background options on Mozilla, but it makes no difference.
I am using an HP Think Pad Laptop, running Windows XP SP2.

Any suggestions on how to fix this would be appreciated.