The Web Developer Firefox add-on has so many features, I'm still finding new ones that I didn't realize existed. After you install the add-on, you will have a new toolbar that you can enable through the View -> Toolbars menu:

After you enable the Web Developer toolbar, you will see the following toolbar added to your browser:

Each toolbar item has a submenu with lots of features.

You can see why I may not know about all of it's features.

There are also a few message icons to give you quick information about the page you are currently viewing. One gives information about standards compliance, the next one give information about CSS errors, and the last one gives information about Javascript errors. Clicking on these icons will bring up more information.

SO! Which features do I regularly use?

• The Cookies menu is a big one. Using this, I can quickly view, edit, and delete my cookies.
• I use Forms -> View Form Information a lot when working with form data.
• I just looked at the Images menu items and I think there are some in there I'm going to have to test out soon.
• I use Resize when I'm about to do a video capture to set my browser to the appropriate size.
• I use the Javascript Errors message icon constantly to quickly pop open the Error Console.