If you are doing web development, you have to get the Web Developer extension to Firefox. It makes developing pages much easier.

Some of the great features:

- It offers a way to view the source of the generated page, even after the page has been altered by AJAX calls. If you are doing AJAX development, this is a must-have so that you can see the state of your page after AJAX calls are made.

- The ability to outline HTML elements on the page, on the fly without altering your CSS. This makes CSS trouble shooting a lot easier.

- The ability to resize your browser to standard sizes

- The ability to switch between CSS box models. This makes developing for other browers (IE) much easier.

The list goes on. If you are doing web development, do yourself a favor and take a look at this Firefox extension.