Like this? Firebug's giving me an error:

document.getElementById(tabid) is null
var root = document.getElementById(tab...ntsByTagName('tbody')[0]; //the root

Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script type="text/javascript">
/*<![CDATA[*/
function addRow(tabid){
var root = document.getElementById(tabid).getElementsByTagName('tbody')[0]; //the root
var allRows = root.getElementsByTagName('tr');//the rows' collection
var cRow = allRows[0].cloneNode(true)//the clone of the 1st row
var cInp = cRow.getElementsByTagName('input');//the inputs' collection of the 1st row
for(var i=0;i<cInp.length;i++){//changes the inputs' names (indexes the names)
cInp[i].setAttribute('name',cInp[i].getAttribute('name')+'_'+(allRows.length+1))
}
var cSel = cRow.getElementsByTagName('select')[0];
cSel.setAttribute('name',cSel.getAttribute('name')+'_'+(allRows.length+1));//change the selecet's name
root.appendChild(cRow);//appends the cloned row as a new row
}
function shownames(){
var allInp=document.getElementsByTagName('input');
for(var i=0;i<allInp.length;i++){
alert(allInp[i].name)
}
}
/*]]>*/
</script>
</head>
<body>
<form action="" method="get">
  <table width="766"  border="0" cellspacing="0" cellpadding="0" id="myTab">
    <tr>
      <td width="191"><input type="text" name="textfield_a" /></td>
      <td width="191"><input type="text" name="textfield_b" /></td>

      <td width="98"><select name="select">
        <option value="item1" selected="selected">item1</option>
        <option value="item2">item2</option>
        <option value="item3">item3</option>
        <option value="item4">item4</option>
        <option value="item5">item5</option>

      </select></td>
      <td width="286"></td>
    </tr>
  </table><br />
  <input name="addARow" type="button" value="Add a Row" onclick="addRow('mytab1')">
  <br />
  <br />
  <input name="submit" type="submit" value="Submit" />
        <br />
        <br />
<input name="button" type="button" onclick="shownames()" value="Show names">
</form>
</body>
</html>