fixed table with header visible
===

I have two tables the structure is below ,the first table is for the header,the 2nd table for 
data .,i get vertical scroll and that is fine.
When i scroll i need that header to be visible along with data (header :fixed).
This is working fine in firefox 3.6 version,but in firefox 11,12,13 it is hiding the header .
Below is the code ,please let me know



    <table  cellspacing="1" cellpadding="5" width="90%" bgcolor="#538ED5" 

    style="height:        300px; overflow-y: fixed">

    <tr>

    <th colspan="2" align="center" bgcolor="#FFCC99" style="font-size:'.$textsize.';" 


    ><b>Dates</b></th>


     <th colspan="3" align="center" bgcolor="#CCFF99" style="font-size:'.$textsize.';" 

      ><b>EACM</b></th>


      <th colspan="3" align="center" bgcolor="#CCFF99" style="font-size:'.$textsize.';" 

     ><b>LOIS</b></th>


     </tr>


      <tr>


    <td >data1</td>



    <td >data2</td>

 
    <td >data3</td>


     </tr>


      </table>