If you have ever developed websites you have to get this Firefox add-on. This tool bar can turn on and off anything such as Javascript, CSS, Images and more. Basically you can test for anything to see if your webpage is stable when people have all those turned off. The toolbar has other features that are handy too. You can test for pretty much anything. Its also great for debugging too.

Image of Web Developer Toolbar

If your a web developer I would download the Web Developer Toolbar for Firefox. This is a good add-on.