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.

Mozilla Firefox is truly the best browser for web designers and developers. It’s Extensions system makes it possible to equip your browser with tools to make your life easier. But if you’re reading this, you probably already know that.

What extensions are the most useful for webmasters?

• ColorZilla - Like having Photoshop’s Eyedropper tool in Firefox.
• Lorem Ipsum - Generate Pseudo-Latin dummy text.
• Firebug - “edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.”
• IE View - Open the current page in Internet Explorer.
• MeasureIt - Drag a ruler across your browser’s viewport.
• Web Developer Toolbar - One of my favorites, this extension adds dozens of useful options to your browser’s arsenal. You can disable JavaScript and CSS, resize your window to exact dimensions, and more. Here’s one of my favorite parts: Press Ctrl-Shift-F to toggle a “Element Inspector” tool. Forget the DOM Inspector, this is more useful.
• YSlow - How fast is your website? How can you improve it? Requires Firebug.

Note: This list is in no meaningful order.