Image map only lines up with header in certain browsers?

Alright, please answer in very simple terms, because I am utterly clueless.

I created an image map for my header, and it only lines up with the tabs in certain browsers (Google Chrome, for example.)
However, in Mozilla Firefox and IE7 the links are skewed off to the left of the image tabs. Is this a known issue? Is there an easy fix? I’m completely stumped, and my ignorance on the subject doesn’t help matters any.