How to X-ray your webpage
I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
Here's a nice extension (sorry, add-on) to Firefox that I find really useful. It's called X-Ray by Stuart Robertson and it enables you to see the structure of any webpage in your browser without switching to view source mode.
The X-Ray add-on provides an extra command on the right click menu of Firefox. Simply, right click, choose the X-Ray option, and the webpage structure is displayed on top of the page you are viewing. HTML element, class names and id names are displayed which makes it easy to see how the page is constructed. It works great!