Add HTML Elements to XUL Addons

I've been working on a new addon at Mozilla which isn't anywhere near finished so I wont bother telling you what it's meant to do...because it could change at any moment.  Since this is my first real addon, it's been a struggle, but that's a good thing -- it means I'm learning a ton.  One problem I ran into was trying to get a checkbox (INPUT) element within the addon via JavaScript -- nothing was displaying.

After a bit of research and guesswork, I figured out what I was doing wrong -- I was using the wrong document method to create the element:

var input = document.createElementNS("http://www.w3.org/1999/xhtml", "input");

Using document.createElementNS with the proper namespace resulted in my checkbox displaying in all of its glory.  It's beautiful square, checked glory.  Firefox addons can be created in a number of ways so if you're looking to create your own and you aren't seeing HTML elements correctly, keep this solution in mind!