Do you know why there are some differences between IE and FF on this page, like these : - logo image on top is not centred on ie - div "block-block-1" with "Ecrivez au Webmaster des Tontons" is not printed on the right place - ...
Which versions of IE and Firefox are you testing against? The site looks identical in IE 8 and Firefox 3.5.
"Ecrivez au Webmaster des Tontons" is invisible because it's in a <p> with 1.5em top and bottom margin. However, <div id="header-region"> is set to be 20px tall, with overflow: hidden. The margin is greater than 20px, so the text is pushed down out of view.
<div id="logo-title"> is centered correctly in IE7 if you remove the width:800px. You've also got a left:50% which doesn't appear to do anything.