Yes, really weird. Firefox is picking up your slider and shifting it to the top of the page, thereby covering your logo. And no coaxing in Firebug will bring it back on top. Safari leaves your header in tact, with the standard bottom grey border with your logo on its white background.
But I’m on FF 26, which has been around since December. Last week was when your problem started, and 27 was released last week, but I’m running 26.
Could you post your full style.css and functions.php (in backticks) here?