Onclick Does Not Work In Firefox?

I am trying to use onclick to add text to a textarea, the function I am using is:

function addtxt(input,add) {
var obj=document.getElementById(input)
var txt=document.createTextNode(add)
obj.appendChild(txt)
}

and the line that I use to call the function - through php- is:

<a onclick="addtxt('post','$code');">
<img src="smiles/$smile" alt="$smilename" border="0" style="cursor: pointer;"></a>

it seems to work in IE but not in firefox, why is that?