Firefox has long been recognized as the best browser in the professional world. We are privileged to extend the capabilities of firefox  by installing various useful add-ons of our choice.
Here we list the most finest and significant add-ons that all users should be aware of.
DownThemAll! – The best and only download manager/accelerator built inside Firefox!

Download Statusbar – View and manage downloads from a tidy statusbar – downloads are no longer managed in a separate window

Duplicate Tab – It allows us to clone a tab

Tab Scope – Preview and navigate tab contents as popup when you place the mouse on the tab.

SwitchProxy Tool -Using SwitchProxy we can manage and switch between multiple proxy configurations

Foxmarks Bookmark Synchronizer – synchronize bookmarks among your computers (if you use multiple computers)

Firebug – You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page

Hyperwords – You can select any text on any web page

Fast Video Download – The best tool to download embedded video files from sites like YouTube, Dailymotion, Metacafe and more on one click!

Download Embedded - Downloads all or selected embedded objects on a webpage. Can be used for downloading movies, mp3s, flash, quicktime, or other embedded files (does NOT work on video embedded in flash such as youtube)

CoolPreviews – Preview the page by just placing the mouse pointer on it

Boost for Facebook – Best tool to deal with facebook!

Flagfox – Displays a country flag depicting the location of the current website’s server and provides quick access to detailed location and webserver information

Gspace – This extension allows you to use your Gmail Space (7.2 GB and growing) for file storage (just like an online storage system)

Dictionary Tooltip – Press ctrl+shift+D (or) double-click (or) right-click after selecting a word to see its meaning.

GooglePreview - Inserts preview images (thumbnails) and popularity ranks of web sites into the Google and Yahoo search results pages.

IE Tab – Embedding Internet Explorer in tabs of Mozilla/Firefox (works with windows only)

Clipmarks – Instead of copying and pasting links it lets you to capture exactly what you want others to see (text, images or video)

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

All-in-One Sidebar – It lets you quickly switch between sidebar panels, view dialog windows such as downloads, extensions, and more in the sidebar, or view source or websites in the sidebar

Yoono – Socialize your browser – Bring all your friends in your browser with Yoono and..
- Follow what they are doing
- Chat with them wherever they are
- Discover and share any content by a single drag & drop
Support: MSN, Yahoo, Gtalk, AIM, Facebook, MySpace, iMeem, Flickr, Twitter, Friendfeed

CustomizeGoogle – It is a Firefox extension that enhance Google search results by adding extra information (like links to Yahoo, Ask.com, MSN etc)

Surf Canyon – Search Engine Assistant – Find stuff you need faster on Google, Yahoo, MSN and Craigslist. Uncover relevant information from as deep as page 100 of the search results. Personalize your search in real-time.

If you come across any other better add-ons, you are welcome to post them here as a comment.