Mozilla Firefox is a very popular browser, especially among web designers and developers. One of the many compelling reasons why Mozilla Firefox is by far the most used browser by web developers and designers is it’s variety of Add-Ons that come free, can be easily integrated and used right from the browser. Mozilla has a complete section for all kinds of Add-Ons available at Firefox Add-Ons page.

Among the massive list of add-ons, many are vital to web designers and developers. If used, it will make your workflow quicker and more productive. Let’s look at our top 20 essential Firefox extensions geared for people who build websites.

1. Web Developer

The Web Developer add-on extends Mozilla Firefox by adding a toolbar with various options for working with web pages. For example, it has an option to inspect CSS styles of page elements that will tell you what CSS attributes they have, and which styles affect them.

Web Developer Firefox AddOn

2. Firebug Firefox Extension

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…

Firebug Firefox Extension

3. IE View Firefox Addon

This application lets you to load pages in internet explorer or even to mark certain pages to be loaded automatically in IE.

IE View Firefox Addon

4. SeoQuake SEO extension

Seoquake is a Firefox SEO extension aimed primarily at helping web masters who deal with search engine optimization(SEO) and internet promotion of web sites. Seoquake allows to obtain and investigate many important SEO parameters of the internet

SeoQuake SEO Extension

5. YSlow by Yahoo! Inc

YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages. YSlow is a Firefox add-on integrated with the Firebug web development tool. YSlow grades web page based on one of three predefined ruleset or a user-defined ruleset. It offers suggestions for improving the page’s performance, summarizes the page’s components, displays statistics about the page, and provides tools for performance analysis, including Smush.itâ„¢ and JSLint.

YSlow by Yahoo! Inc

6. Alexa Sparky Firefox Add-On

Sparky is Alexa Internet’s free Firefox status bar plugin. Sparky accompanies you as you surf, providing you with Alexa data about the sites you visit without interrupting your browsing.

Alexa Sparky Firefox Add-On

7. Window Resizer Best Firefox Add-On

The Browser Window Resizer is useful for testing different screen sizes. It accurately resizes your browser so you can test to see what a web page looks like in all of the standard resolution sizes. Supports the 640×480, 800×600, 1024×768, 1280×800, 1280×1024, 1600×1200 resolutions. Resize the current page via the Context menu, Tool menu or Toolbar button.

Window Resizer Best Firefox Add-On

8. CSSViewer Firefox Addon

It’s really handy for web designers to inspect individual CSS elements of a webpage especially when designing pages or taking inspiration from a design or even when you just want to do a straight copy too. CSS Viewer integrates into Firefox and lets you inspect each CSS element when you simply hover over it. You may turn it off when you are just browsing and don’t want to inspect CSS elements of the webpage.

CSSViewer Firefox Addon

9. ColorfulTabs Extension

The most beautiful yet the simplest add-on that makes a strong colorful appeal. Colors every tab in a different color and makes them easy to distinguish while beautifying the overall appearance of the interface. For Seamonkey & Flock. An essential.

ColorfulTabs Extension

10. FireShot Add-On for Firefox

FireShot is a Firefox extension that creates screenshots of web pages (entirely or just visible part). Unlike other extensions, this plugin provides a set of editing and annotation tools, which let users quickly modify web captures and insert text annotations and graphical annotations. Such functionality will be especially useful for web designers, testers and content reviewers.

FireShot Add-On for Firefox

11. FireFTP – FTP client for Mozilla Firefox

Along with transferring your files quickly and efficiently, FireFTP also includes more advanced features such as: directory comparison, syncing directories while navigating, SFTP, SSL encryption, search/filtering, integrity checks, remote editing, drag & drop, file hashing, and much more!

FireFTP - FTP client for Mozilla Firefox

12. ColorZilla Firefox Extension

Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies. With 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. DOM spying features allow getting various information about DOM elements quickly and easily. And there’s more…

ColorZilla Firefox Extension

13. Firebug – Very Useful Firefox Add-On

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…

Firebug - Very Useful Firefox Add-On

14. FirePHP Firefox Extension

FirePHP enables you to log to your Firebug Console using a simple PHP method call. FirePHP is ideally suited for AJAX development where clean JSON or XML responses are required. All data is sent via response headers. This means that the debugging data will not interfere with the content on your page.

FirePHP Firefox Extension

15. Greasemonkey Add-On

The Greasemonkey Firefox add-on enables you to modify how a web page looks and works by using small snippets of JavaScript. There are hundreds of scripts that you can download to enhance this add-on.

Greasemonkey Add-On Plugin

16. GridFox Extension for Firefox

Draws a grid on top of a website. This is useful for checking designs that are supposed to follow a grid-based layout. Right click anywhere on a website, and go to GridFox > Toggle Grid. In the bottom right, you’ll see some buttons to help you create, edit and save a grid. You’ll also see some arrows which let you browse any existing grids for the site you’re looking at.

GridFox Extension for Firefox

17. Palette Grabber Firefox Mozilla Add-On

Creates a color palette for Photoshop, Paint Shop Pro, GIMP, Flash, Fireworks, Paint.NET, or OS X based on the current page.

Palette Grabber Firefox Mozilla Add-On

18. Live HTTP Headers Firefox Add-On

For dynamic webpages, web developers often have to inspect headers of the page and usually they do it manually by inspecting the source of the returned pages. With Live HTTP Headers Firefox extension, you can view HTTP Headers as they come in while the page is loading.

Live HTTP Headers Firefox Add-On

19. Font Finder Firefox Extension

FontFinder is created for designers, developers and typographers. It allows a user to analyze the font information of any element on a page, copy any piece(s) of that information to the clipboard, and perform inline replacements to test new layouts.

Font Finder Firefox Extension

20. Total Validator Add-On for Firefox

All new HTML5 and XHTML5 validation. Provides true HTML validation for HTML 2.0 through to XHTML 1.1 using the official DTDs, plus added attribute and other tests. So you no longer have to put up with the limitations of the W3C validator (no type checking) and tools like HTML Tidy which interpret the standards incorrectly as they don’t use the official DTDs.

Also performs accessibility validation (WCAG 1/2, US-508), broken link checking, spell checking (5 languages), and takes screenshots using numerous browsers on Windows, Linux, and OS X.

Total Validator Add-On for Firefox

21. PageDiff Firefox Add-On

Pagediff is a simple page compare application. It helps web developers and designers to see HTML-code(text) differences between web pages.

PageDiff Firefox Add-On

22. Codetch Mozilla Add-On

Get the feel of Dreamweaver in a Firefox extension. Edit your documents right next to your web pages as you surf.

Codetch Mozilla Add-On