Web Developer
Web Developer adds a handy toolbar with lots of options that are useful for web developers.

IWebTool
A webmaster’s toolbar with over 30 different tools.

Load Time Analyzer
The Load Time Analyzer allows developers to measure and graph how long web pages take to load in Firefox.

Firebug
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.

CSSViewer
A simple add-on that lets you view CSS properties quickly and easily.

EditCSS
Allows you to edit the stylesheet right in your sidebar.

CSS Validator
With one click you can quickly validate a page using the CSS validator from W3C (opens results in a new tab).

Validaty
One-click validation from W3C’s validator.

HTML Validator
Adds HTML Validation information inside the browser as you are viewing pages. Shows the number of errors for each page.

Total Validator
This add-on performs multiple validations and takes screen shots.

LinkChecker
Checks the validity of links on any page.

View Formatted Source
Displays formatted and color-coded source and optional CSS information for each element.

Live HTTP Headers
While browsing, Live HTTP Headers allows you to view the HTTP headers of a page.

Modify Headers
With Modify Headers you can add, modify, and filter HTTP request headers.

Professor X
Want to see inside a page’s head without viewing the source code? Try Professor X.

SourceEditor
View and edit the source HTML.

View Source Chart
View Source Chart draws a color-coded chart of a page’s source code.

CSSMate
CSSMate allows inline CSS editing (originally was a part of EditCSS. When you wish to modify the CSS of a page, press Ctrl+Shift+8.

ViewSourceWith
Primarily for viewing page source with external applications, but there are a lot of other options as well.

Font Finder
With Font Finder you can easily get all the styles of selected text. All you have to do is highlight text, right-click and select Font Finder.

Accessibar
Accessibar is a web usability toolbar. It enables easy manipulation of web page display and text-to-speech output.

Aardvark
Aardvark is a very useful add-on if you’re looking to control how a page prints. You can remove elements that you do not want to be printed.

Copy as HTML Link
Save yourself a little bit of time by automatically creating an HTML link to the current page that can be pasted into another application.

YSlow
YSlow will analyze a page that is loading slowly and tell you why, based on Yahoo’s rules for high-performance websites.

FaviconizeTab
If you use a lot of open tabs while you’re working online, this add-on can help you compress them by showing only the favicon when the tab is not selected.

FireFTP
A free and secure FTP add-on.

Open IE
A great add-on for web designers, allows you to open a tab in Firefox that displays the page as it would appear in Internet Exporer.

OperaView
Open pages in Opera from Firefox and Mozilla context menus. Very useful for web developers.

ColorZilla
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.

MeasureIt
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.

Venkman
Venkman provides a powerful JavaScript debugging environment for Mozilla based browsers namely Firefox 2.x, the Netscape 7.x series of browsers, Netscape 9.x series, Mozilla Seamonkey 1.x and Mozilla Seamonkey 2.x.

XMLDeveloper
Provides a toolbar to help XML developers with their programming efforts by supplying a central toolbar for everything XML.

Screengrab!
An extension for Firefox that makes it easy to save a web-page as an image. With it, you can save anything that you can see in a browser window - from a small selection, to a complete page.

ImageBot
ImageBot allows lets you upload your images to a free image hosting service (ImageShack or Photobucket).

Save as Image
Adds the ability to save a page, frame, or part of either as an image.

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

ScribeFire
ScribeFire (previously Performancing for Firefox) is a full-featured blog editor that integrates with your browser and lets you easily post to your blog. You can drag and drop formatted text from pages you are browsing, take notes, and post to your blog.

Social Media Extension
This extension will save you time if you use social media sites like StumbleUpon, Digg, Reddit, and del.icio.us. You can quickly see if a page has been submitted to these sites, which will allow you to find great content and be the first to submit it.

StumbleUpon Toolbar
Have StumbleUpon’s feature accessible right from your toolbar.

Del.icio.us Toolbar
Have the features of del.icio.us accessible from your toolbar.

Source:vandelaydesign