xanadu-fx.com

One way to save a lot of time when designing or debugging web sites is to add the Web Developer Extension to Firefox. It consists of a toolbar and menu with several great features.
With it installed, you can edit style sheets (or HTML) in real time, which saves a lot of FTP and preview time.
Also, you can display just about anything you can imagine such as Id and Class details:

The extension also has an “outline” feature (screen shot) - tables, cells, block level elements and a lot more:

Other features allow the user to hide or outline images, resize the browser to 800×600 or other custom sizes, and use the W3C validation tools with one click:

The Web Developer Extension can be found at chrispederick.com