How to insert a line break with javascript?

You should create new <br> tag each time when you will append it, something like

linebreak = document.createElement("br");
queryForm.appendChild(linebreak);

DEMO

The problem with your code is you're inserting the same element over and over again.

Here's an analogy: Imagine you have several children lined up in a row. Yes you can attempt to move one of them after each of the other children, however, at the end of the day you still only have one of that child. He's going to end up at the end of the row.

Solution: Create the line break element each time you need to insert it.

Also, here's an obligatory plug for jQuery: http://www.jquery.com