replied by accident...

The DIV above it with the ads have floating elements. In such a case you need to add a clearing DIV to prevent content from being placed to the right of it. You can either add an extra DIV with style="clear:both" under the last DIV.ai (preferred). In that case you need to remove the height:35px from the containing DIV as well (width: 750px; height: 35px;). Or you can add the style="clear:both" to the DIV with the shifted content (<div style="text-align: center; clear:both">).

You should always be careful with specifying the height. It is always better to let the browser do this in case of increased font sizes or other changes made by visitors.

<div id="1974125074" align="center">
 <div class="bnu"></div>
 <div style="width: 750px;">
  <div class="ai" style="margin-right: 5px;"></div>
  <div class="ai" style="margin-right: 5px;"></div>
  <div class="ai" style="margin-right: 5px;"></div>
  <div class="ai" style="margin-right: 5px;"></div>
  <div style="clear: both;"></div>
 </div>
</div>

<div style="text-align: center;">
 <div style="width: 900px; margin: auto;">
  <div>
   <table>
   </table>
  </div>
 </div>
</div>

thank you so much!!! you are a lifesaver.

Please click the Helpful button next to the answer that solved your Firefox support issue, when you are logged in, so this thread gets marked as Solved.