This website renders a background jpg differently than it did in Firefox 5 and below on Windows 7. I know that’s a bit confusing so I’ll outline it.
It works:
All versions of firefox, chrome, and IE on windows XP
All versions of chrome, IE, and firefox 5 and below on windows 7
Example:

It doesn’t work:
Firefox 6 and above on Windows 7
Example:

The website is repeating the gradient jpg horizontally. It then tries to make the background color the same color as the bottom of that gradient.
Thanks again SO.
On my Windows 7 / FF6, the gradient looks pretty smooth:
so I think that what you’re seeing is machine specific.
However, you might have better results by using a .GIF with a transparent bottom (to fade to background color) rather than a .JPG. In general, you’ve got better control over the colors in a .GIF because it’s not a lossy format if you have few enough colors.