These days Mozilla Firefox is becoming more and more powerful tool. Firefox Addons and plugins make it more powerful tool, For Designers and developers each of these extensions can make certain things easier , because searching for errors and optimizing your pages can be pretty painful and time-consuming process. Here we are compiling a list of 40 Best Firefox Addons for Web Designers and Developer.
Greasemonkey
Greasemonkey allows you to customize the way a web page displays using small bits of JavaScript.

Cooliris
Cooliris is the 3D Wall that speeds up search on Google Images, Bing, YouTube, and more. Enjoy the richest way to view photos from Flickr, Facebook, Picasa, and even your computer.

FireFTP
FireFTP is simply an FTP client built into the Firefox browser. Important thing it is free and you can flip to it just like any tab in your browser.

Aardvark
If there is a need of utility for selecting elements and performing various actions then Aardvark is the best choice. It helps in cleaning up a page prior to printing it, for making the page more readable, and for analyzing the structure of a page.

Firebug
Firebug includes a ton of web development tools to use from your browser. You can explore, edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

IE Tab
Pretty handy extension to see how your site is displayed at Internet Explorer.

ColorZilla
ColorZilla helps you in color reading from any point in your browser, quickly adjust this color and paste it into another program. You can also Zoom the page you are viewing,measure distances between any two points on the page, choose colors from pre-defined color sets and saving the most used colors in custom palettes.

HTML Validator
This plugin displays the number of found errors of the site you are viewing. It adds HTML validation inside Firefox and Mozilla.

Web Developer
The Web Developer addon / extension adds a menu and a toolbar to the browser with various web developer tools.

SEO for Firefox
Who like to watch statistics right into Google and Yahoo search results then it is an Excellent plugin for them. You can get Alexa, Google PageRank, Tehnorati rankings and many other useful data.

CSS Viewer
With this extension you are able to view the css properties.Ever been on a site that had some nifty CSS tricks that you loved and wished you knew how they did it?

Measure It
This extension gives you the ability to draw out a ruler to get the pixel width and height of any elements of a web page.

StumbleUpon
If you are a web-developer, StumbleUpon can drive a huge amount of visitors to your website.StumbleUpon helps you to find new sites very easy.

Window Resizer 1.0
It quickly and accurately resizes your browser so you can test to see what a web page looks like in all of the standard resolution sizes. Supports all major screen resolutions from 640×480 up to 1600×1200.

YSlow
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.

Page Diff
This extension helps web developers and designers to see HTML-code(text) differences between web pages which can assist in fixing coding irregularities.

Delicious Bookmarks
Delicious bookmarks is official Firefox addon for del.icio.us. It integrates your browser bookmarks and tags, to keep them in order and “easy to find” way.

Tab Mix Plus
This plugin provides better management of the many firefox tabs opened at the same time.

FirePHP
FirePHP is ideally suited for AJAX development where clean JSON or XML responses are required. FirePHP enables you to log to your Firebug Console using a simple PHP method call.

Unhide Passwords
Unhide Passwords is a tool for everyone who is bad at remembering or typing passwords correctly.

Dummy Lipsum
Dummy Lipsum Firefox Addon/ Extension generates “Lorem Ipsum” dummy text (the text is taken from the Lorem Ipsum site). I found it to be pretty useful as web designer.

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

JavaScript Debugger
A powerful JavaScript debugging environment for Mozilla based browsers.

IE NetRenderer
This adds buttons, tools menu and contextual menu entries to get a screenshot of the current page.It is really useful for webmasters which are not using Windows!
Keyboard shortcuts are also available: Ctrl+Shift+F5/F6/F7/F8 to render the page in IE5.5/6/7/8 Beta 2 (Cmd+Shift+F* on Mac).

FasterFox
Fasterfox allows you to tweak many network and rendering settings such as simultaneous connections, pipelining, cache, DNS cache, and initial paint delay.

FEBE
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox Extensions. In fact, it goes beyond just backing up — It will actually rebuild your extensions individually into installable .xpi files. Now you can easily synchronize your office and home browsers.

Palette Grabber
This extension will create a color palette exportable to all major image editing apps, including, Photoshop, Paint Shop Pro, Fireworks and others based on the current page.

Morning Coffee
This extension lets you organize websites by day and open them up simultaneously as part of your daily routine. This is really handy if you read sites that update on a regular schedule (like webcomics, weekly columns, etc.)

Platypus
Platypus is a Firefox extension which lets you modify a website from your browser. In addition to that you can save your modifications as a Greasemonkey script to make them “permanent”!

Panic
panic Firefox Extension gives you the ability to close all tabs, while opening another one that will make it look like you are doing what you should be doing.

Autofill Forms
Autofill Forms enables you to fill out web forms with one click or a keyboard shortcut.

Selenium IDE
Selenium IDE is an integrated development environment for Selenium tests. It is implemented as a Firefox extension, and allows you to record, edit, and debug tests.

URL Fixer
It corrects typos in URLs that you enter in the address bar. By right-clicking on the address bar, you can set it to auto-correct your errors, or you can have it ask you before making any corrections.

Opera View
Extension based on IE View. If you want quick view in opera, just right click somewhere on page and choose “View This Page In Opera”.

Font Finder
Font Finder simply highlight a single element (e.g. a paragraph or a strong tag ), right-click and select `Font Finder`, then marvel as the full CSS text styling of the selected element appears.

FireShot
FireShot provides a set of editing and annotation tools, which let users quickly modify captures and insert text and graphical annotations.

LiveHTTPHeaders
LiveHTTPHeader allows you to see which kind of web server the remote site is using and cookies sent by remote site.

NoScript
It is good choice for blocking JavaScript, Java and other executable content in certain pages You don’t trust.

W3C Offline Page validator
The web developers nightmare – Validating. Offline Page Validator is a simple tool that will send your page to the W3C validator by simply clicking an option on the right click menu, works both online and offline.

Style Sheet Chooser II
Style Sheet Chooser II allows you to choose author-provided alternate style sheets for a web site, and have those choices be persisted for all pages on the same site.

JS View
JS View which nests itself next to the quick search bar (although at work it sits in the status bar – maybe the versions differ, didn’t check) and when clicking it, it gives you a list of the currently linked stylesheets and javascript files.
