I've just noticed when loading my client's page (http://habbopfm.com/) in Firefox, that it renders what looks like a rectangle is pushed down a few pixels.

It's difficult to describe, but imagine you took a screenshot of the page, opened it in Photoshop, selected a rectangle and moved it down a bit.

IE and Safari don't appear to do this - I'm assuming this is a browser glitch, not a problem with the code. Can anyone confirm what causes this?

Below is a screenshot taken of the issue:

http://habbopfm.com/problem.png

One other thing is that it only does this once the page has finished loading. While it is loading, it looks fine.