Firefox web browser by Mozilla is great. Here is a list of Firefox add-ons that will unleash the power of this popular application. All the add-ons are compatible with the Firefox 3.0.5. Don't miss to install at least these: McAfee Site Advisor, WOT, AdBlock Plus, and Download them All.

Staying safe

Web of Trust

WOT, Web of Trust, warns you about risky websites that try to scam visitors, deliver malware or send spam. Protect your computer against online threats by using WOT as your front-line layer of protection when browsing or searching in unfamiliar territory. WOT's color-coded icons show you ratings for websites - green to go, yellow for caution and red to stop – helping you avoid the dangerous sites.

McAfee Site Advisor

It helps you stay away from dangers lurking on the web by giving you red, yellow and green lights--both for the web pages that you open and also for the links in your search pages. Is also available as a plug-in for Internet Explorer.

NoScript

The best security you can get in a web browser! Allow active content to run only from sites you trust, and protect yourself against XSS and Clickjacking attacks.

Interclue

Get to know what's behind the link before you click. Interclue tells you everything you need to know before you open yet another tab.

KidZui

KidZui turns Firefox into fun, kid-safe browser with over a million kids games, YouTube videos, and websites.

Utilities

Adblock Plus

Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them.

Bookmark This Plus

As the name suggests, it improves the bookmark functionality of Firefox.

Open IT Online

Ever faced problem opening any file on a computer because a particular application is not installed. Open IT Online allows you to open your documents and images online without the need of any software to be installed.

Picnik

Do you use online image editing app Picnik? Then this add-on will save you some steps by directly importing images from the web page for image manipulation to the app site. Add a menu options and a toolbar icon to make importing images into Picnik easier. This add-on has been struck out because it's a list of 45 add-ons, not more.

Surf Canyon: Search Assistant

Surf Canyon accelerates the process of finding relevant information buried in the search results on Live Search, Google, Yahoo! and Craigslist. Available for download as an add-on for both Firefox and Internet Explorer (IE6 and IE7), the software figures out what you want and then automatically recommends relevant search results.

Gspace

This extension allows you to use your Gmail Space (4.1 GB and growing) for file storage. It acts as an online drive, so you can upload files from your hard drive and access them from every Internet capable system.

BabelFish Instant Translation

Get the Web in your own language in a really easy way.

PDF Download

It will enable you to download web pages as PDF. Gain control of PDFs and eliminate browser problems, view PDFs directly in Firefox as HTML, and use the all-new Web-to-PDF toolbar to save and share Web pages as high-quality PDF files. The conversion takes place online, consuming bandwidth.

pdfit

Another PDF add-on which allows user to convert current page to image file, or PDF format (image to pdf). The entire conversion is offline but it annoy you by opening a webpage.

Foxmarks

If you use Firefox on more than one computer, you'll want Foxmarks. Install Foxmarks on each computer, and it works silently in the background to keep your bookmarks and (optionally) passwords synchronized.

Auto Shutdown

Auto Shutdown will enable Firefox built-in download manager to shutdown, hibernate or standby your PC.

Download Statusbar

View and manage downloads from a tidy status bar - without the download window getting in the way of your web browsing.

Video DownloadHelper

The easy way to download and convert Web videos from hundreds of YouTube-like sites. This works also for audio and picture galleries. It supports a large number of video sites.

DownThemAll!

The first and only download manager/accelerator built inside Firefox! It is loved by its users. But then I find it bit problematic--it would not work in background but will show up in my task bar. It should have supported working in system tray without cluttering my wrking space.

FoxyProxy

FoxyProxy is an advanced proxy management tool that completely replaces Firefox's limited proxying capabilities. It offers more features than SwitchProxy, ProxyButton, QuickProxy, xyzproxy, ProxyTex, TorButton, etc.

Firefox Universal Uploader

This extension allows to upload/download files from your favorite websites using a simple interface. Below is the list of all websites supported by Fireuploader. In this version, Snaphappi (a digital photo processing website), Slideshare.net (Share your presentations) and Adobe Share has been added. You can make PDF documents using Adobe Share. Upload any word/image document, download it in PDF format.

Stop Autoplay

Disable the autoplay of the embedded music and movie. A nice function to avoid unwanted playback.

Gui:config

This is an extension for advanced preferences. gui:config makes it easier to change preferences that can only be found in the “about:config”.

Well dressed

FoxTab

FoxTab brings innovative and attractive 3D tab management to Firefox. With its five fascinating layouts, it makes tabbed-browsing a really nice experience.

Smart Bookmarks Bar

Not enough place on your bookmarks bar ?... Smart Bookmarks Bar extension comes right here by hiding bookmarks names and only showing icons. Bookmark names will be displayed on mouse over.

Tab Scope

Preview and navigate tab contents through popup.

Better read

Read It Later

Read It Later allows you to save pages of interest to read later. It eliminates cluttering of bookmarks with sites that are merely of a one-time interest.

NewsFox

NewsFox is an efficient 3-pane(email) style Atom/RSS feed reader.

Morning Coffee

Keeps track of daily routine websites and opens them in tabs. Is good if you have a slew of websites that you visit on a regular basis.

Staying connected

TwitterFox

TwitterFox notifies you of your friends' status on Twitter.

Simple Mail

This add-on converts Firefox into a mail client (POP3/IMAP/SMTP).

Brief

Brief makes reading RSS feeds as easy and intuitive as it gets. Feeds are presented on a seamless, interactive page, which let's you bookmark and tag items with a single click.

Undercover agent

Ghostfox

An interesting add-on if you wish to browse discreetly. Ghostfox stylizes Firefox into Ghostzilla (the invisible browser) style. Move your mouse away and Firefox disappears. Sweep to the left of the screen, to the right, and again to the left and Firefox is back, blended with your focused application.

Page Title Eraser

The Page Title Eraser (PTE) is a simple Mozilla Firefox extension which allows to hide title and icon (favicon.ico) of the selected tab. It provides more privacy at your workplace.

Tools for bloggers

ScribeFire Blog Editor

ScribeFire is a full-featured blog editor that integrates with your browser and lets you easily post to your blog.

Deepest Sender

Deepest Sender is a client that will allow you to post to blogs from directly within Firefox. It is primarily a LiveJournal client, although it supports Blogger (GData) and WordPress (metaWeblog) too.

FireShot

FireShot is a Firefox extension that creates screenshots of web pages. 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. It's possible to choose whether you want to capture entire web page or take screenshot of only visible part of the web page.

Add-ons for developers and designers

Greasemonkey

Allows you to customize the way a webpage displays using small bits of JavaScript.

Web Developer

Adds a menu and a toolbar with various web developer tools.

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... Firebug 1.3 requires Firefox 2 or 3.

IE Tab

Open Internet Explorer inside Firefox. Handy for viewing a site in IE for getting rid of design issue or for testing purposes.

ColorZilla

Allows you to pick colour with the help of an eyedropper tool. a handy tool to know hexcode of colours that you like on the web.

MeasureIt

Draw out a ruler to get the pixel width and height of any elements on a web page.

FireFTP

FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.

Html Validator

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. The extension can validate the HTML sent by the server or the HTML in the memory (after Ajax execution) .

User Agent Switcher

Adds a menu and a toolbar button to switch the user agent of the browser.

FYI: It took me at least six [not four] hours to compile this list after going through almost all the pages on Firefox Add-ons site :)

Additional resources:
Firefox Add-ons