Textarea Key Press Disables Dropdown Add In Firefox?

My script works ok in IE but not in firefox.Basically i have a dropdown list that onclick adds the values to a textarea field on same page. It works ok if you just select from the dropdown to begin.However as soon as you press a key with the cursor in focus of the textarea it disables the further addition from the dropdown.This doesn't happen in IE so it's probably a syntax error.'m new to all this and i search the internet to build the scripts by trial and error -this is my way of learning. Anyway here is the code;avascript:

function test(){
document.getElementById('taid').innerHTML+=document.getElementById
('selid').value;

[code]....