Here's a fixed version.

Case sensitivity is important to Firefox.

There's no such attribute as "type" on a <button>, which broke IE.


<h3>Content</h3>
<div id="divTxaTools">&nbsp;</div>
<textarea cols="60" rows="20" id="txaContent"></textarea>

<script>
var txaTools;
var txaContent;

var btnHeightInc, btnHeightDec, btnWidthInc, btnWidthDec;

window.onload = function() {

    txaTools = document.getElementById("divTxaTools");
    txaContent = document.getElementById("txaContent");
   
    btnHeightInc = document.createElement("button");
    btnHeightDec = document.createElement("button");
    btnWidthInc  = document.createElement("button");
    btnWidthDec  = document.createElement("button");
   
    btnHeightInc.innerHTML = "Increase Height";
    btnHeightDec.innerHTML = "Decrease Height";
    btnWidthInc.innerHTML  = "Increase Width";
    btnWidthDec.innerHTML  = "Decrease Width";
   
//    btnHeightInc.type = "button";
//    btnHeightDec.type = "button";
//    btnWidthInc.type  = "button";
//    btnWidthDec.type  = "button";
   
    btnHeightInc.onclick = HeightIncrease;
    btnHeightDec.onclick = HeightDescrease;
    btnWidthInc.onclick  = WidthIncrease;
    btnWidthDec.onclick  = WidthDecrease;
   
    txaTools.appendChild(btnHeightInc);
    txaTools.appendChild(btnHeightDec);
    txaTools.appendChild(btnWidthInc);
    txaTools.appendChild(btnWidthDec);
   
}

function HeightIncrease() {
    txaContent.rows = txaContent.rows + 1;
}
function HeightDescrease() {
    txaContent.rows = txaContent.rows - 1;
}
function WidthIncrease() {
    txaContent.cols = txaContent.cols + 1;
}
function WidthDecrease() {
    txaContent.cols = txaContent.cols - 1;
}
</script>