The Firefox Web developer extension – is by far the most useful tool for Web development using Firefox. This toolbar is packed full of useful web testing tools. It can outline page elements, display the size of images, display CSS and form data, disable certain elements on a page, and more. It really does more than I could explain in this single post. Aardvark Extension – Are you tired of using border:1px solid #000; to debug weird CSS behavior. Well this extension doesn’t completely eliminate the need for the border trick, but it sure does help. Aardvark allows you to you a visible border around items on the page while displaying their id and/or class. You can then press certain keys on the keyboard (as indicated below) to do various things, such as delete the selected element from the page, isolate the element, or move the selection rectangle outward to the containing element. So while The Web Developer toolbar is the most necessary, Aardvark is the one you will use the most Colorzilla – An eyedropper tool for color selection and testing. Hovering over an element shows color info, DOM path info, and the X, Y offset from the previous click. Fang – This extension creates a textual representation of a web page similar to how the page would be read by a modern screen reader. Link Checker – Every year one of the tasks we have to do as AIR judges is check the links on each constestants page. The W3C check works is just, well slow. This link checker checks webpage links at a glance with color coding. Measure It – allows you to measure screen height and width by drawing a ruler across a web page to display the width, height, or alignment of any page element. No more taking a screen grab and pasting it into Photoshop or Fireworks to see how many pixels an object takes up. Screen Grab – SnagIT is by far by favorite screen capture tool because it allows me to do pretty much everything I need to do in one application. There are instances when I need to take a quick screenshot of a web site and SnagIT doesn’t integrate with Firefox. Screen Grab saves webpages as images by scrolling around and stitching the result together. IE View – Every now and then you will run across a page that either requires IE or just looks better in Internet Explorer. This happens to me every time I want to view a video on MSN. This extension allows the current page or a selected link to be opened in Internet Explorer. Note: Another Tool IE Tab allows you to view pages in a separate IE Tab, so that you don’t even have to leave the firefox interface. Fire FTP – I almost never perform bulk FTP operations with Dreamweaver, because it just seems to take too long. FireFTP is a free FTP client for Mozilla Firefox which provides both FTP and SFTP access to servers. Its not as full featured as other tools like WS FTP, but it gets the hob done. HTML Validator – This extension is based on the now famous HTML tidy program . It adds validation inside the Page Source Viewer in Firefox. UPDATE: Firebug – Thanks to Jon for this one. Firebug rocks. It combines a lot of the functions of Aardvark and it a great tool for diving into the DOM and XMLHttpRequest object. I have a feeling I will be using this one a lot too. BONUS: Del.icio.us Extension – Not necessary for web development, but it seamlessly integrates everything you need to use Del.icio.us directly in Firefox.