You can install the extension by simply clicking on the link for it. Near the top of the Firefox window, you may see the message "Firefox prevented this site (chrispederick.com) from asking you to install software on your computer." Click on the Allow button next to the message. When it is installed, you wil be notified you should restart Firefox to complete your changes. Click on the Restart Firefox button on the Add-ons window that appears to restart Firefox. When Firefox is restarted, you should be returned to the page you were on when it restarted (at least when I installed it for Firefox 3.0.1 that is what occurred).
If you are have problems installing the extension then try the following:
Once the extension is installed ou will see a toolbar with Disable, Cookies, CSS, Forms, Images, Information, etc. at the top of the Firefox window (Toolbar).
Within each of the toolbar items are other options. For instance, under Tools, you will see the following options, as well as others:
Validate CSS Validate Feed Validate HTML Ctrl+Shift+H Validate Links Validate Section 508 Validate WAI
Firefox Web Developer Extension
Version: Web Developer 1.1.6
Build Date: May 19, 2008
Author: Chris Pederick
Developer Website: http://chrispederick.com/work/web-developer/
Download from this website
[Note: Current version as of August 5, 2008; you should check the developer's website for the most current version.]