have a look at the top menu on these two pages on Firefox:
On the first page for some reason there is some extra spacing above it.
This only happens on Firefox. I am using Firefox 3.6.
I can see the space in Firefox 3.0.15 on Mac OS X 10.4. A quick glance shows two differences in the pages:
The index.php page has extra styles added by the bidvertiser.com code and an extra style declaration in the head section of the HTML. Perhaps one of those is overriding your main stylesheet(s)?