Web Developer Toolbar is one of my favorite Firefox extensions. It makes design and development so much easier.
It can resize your browser window to a specified size (great for getting a rough idea of how a page will look at a lower resolution), check to see if your HTML, CSS, or RSS validate, easily disable JavaScript or CSS, make hidden elements visible, and much more. There are eleven menus full of useful tools. I don’t use half of them myself, because I forget they’re there…
One of my favorite features is Command+Shift+F (Ctrl+Shift+F on PC). It puts you into a DOM inspection mode where you can click an element on the page and get a variety of details, such as the element ID and class, the DOM path, and a multitude of CSS attributes.
If you do even a little bit of design or development, I guarantee you this add-on will make your life easier.
