Essential firefox add-ons for web programmers

• FireFTP - is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers, FireFTP includes more advanced features such as: directory comparison, syncing directories while navigating, SSL encryption, search/filtering, integrity checks, remote editing, drag & drop, file hashing, and much more!
• IE Tab - is a great tool for web developers, since you can easily see how your web page displayed in IE with just one click and then switch back to Firefox.
• Firebug - integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
  
• The Web Developer - adds a menu and a toolbar to the browser with various web developer tools.
• HTML Validator - is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.
• ColorZilla - you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes.
• CSS Validator - is a Mozilla Firefox extension which Validates a page using the W3C CSS Validator. Adds an option to the right-click context menu and to the Tools menu to allow for easy validation of the CSS of the current page.
• MeasureIt - draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
• View formatted source - displays formatted and color-coded source and optional CSS information for each element.
• View Source Chart - draws a Color-Coded Chart of a Webpage's Source Code and displays Source in its Altered State After the DOM has been Manipulated by JavaScript.
• Venkman - the code name for Mozilla's JavaScript Debugger, aims to provide a powerful JavaScript debugging environment for Mozilla based browsers namely Firefox, Netscape 7.x/9.x series, Seamonkey 1.x/2.x.
• iWEBTOOL - Access over 30 Webmaster Tools for Free directly from your web browser
• http://livehttpheaders.mozdev.org/
• LinkChecker - check webpage links at a glance with simple color coding. Ditch those massive listings of bad links that provide no context and add LinkChecker to your arsenal of web development tools today.
• Load Time Analyzer is an extension created by Google that displays the number of events that are processed by a certain website and how long it takes to load them all. The load time displayed will obviously be influenced by your Internet connection, nonetheless it can provide a good indicator of the site speed.
• Quick Locale Switcher - Allows you to quickly change and apply a different locale (language) in your Mozilla application.
• XHTML-MP - allows firefox to render HTML content with the mime-type of application/vnd.wap.xhtml+xml. For any developer working with mobile applications, this provides an alternative to using a device emulator.
• Console² - let's you display errors filtered by type (Errors, Warnings, Messages), language (JavaScript, CSS, XML) and context (Chrome, Content). Furthermore it provides a simple search box (as seen in the History and Bookmarks sidebars), hiding of duplicates, sidebar optimizations, a customizable toolbar and some more accessibility improvements.

• Page validator - validates a page using the W3C Markup Validation Service. Adds an option to the right-click context menu and to the Tools menu to allow for easy validation of the current page. Opens the results in a new tab. This is a simple extension that will work only for online pages.
• xmldeveloper - helps XML developers with their programming efforts by supplying a central toolbar for everything XML.
• The Total Validator - provides one-click validation. Once installed all you do is browse to the page you want to validate then click on the 'TV' icon in the browser status bar.
• Modify Headers - allows the user to Add, Modify and Filter out HTTP request headers. See the help tab in the Modify Headers window for more information.
• The XPather - is a simple extension that integrates both with the browser and its DOMInspector. Thus, is't very lightweight and cross-platform. It is valuable mainly as a web/XML-app development and hacking tool.
• LiveHTTPHeaders - display http headers in real time while pages are being downloaded from the Internet
• Font Finder - Get all CSS styles of selected text in Firefox