My Essential Firefox Plug-Ins

The more I do Web development, the more I wonder how I ever built any applications without Firefox and its many plug-ins. Here are my essentials:

Firebug
I'm sure I'm not taking advantage of all the great features of this plug-in. As I do more and more jQuery development, the JavaScript console has become a necessity. Also great for debugging AJAX. The inspector is great for working CSS issues.

MeasureIt
Simply drag out a rectangle out the screen and you can instantly get the dimensions of any object on the screen.

ColorZilla
Found a color on the screen that you need? Use the dropper to find it, then right-click the picker to copy the color code in various formats (hex, rgb, hsl).

Live HTTP headers
This plug-in will show you all of the HTTP headers during a page load. I once used this tool to figure out that the reason a particular page was broken was because a key element being included was blocked by the company firewall.

Web Developer
Drop down menu includes: Disable, Cookies, CSS, Forms, Images, Information, Outline, Resize, Tools, and more. Let's you do all sorts of manipulation for debugging Web development.

What about you? What are your "must haves"?