Because of success of open source in nature, there are more addons available for Firefox. That makes the live of developers, webmasters, and all the people who live in Internet.

Here I have listed some of best Firefox addons you can add and use inside Firefox.

Search Status:
Addon for Mozilla Firefox browser. Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and other SEO tools.

Search Status firefox addon

Xmarks
Xmarks is the #1 bookmarking add-on. It takes only a moment to get up and running with Xmarks. After you install the add-on, click on the notification to set up Xmarks and start backing up and synchronizing your bookmarks. Install Xmarks on each computer you use, and it seamlessly integrates with your web browser and keeps your bookmarks safely backed up and in sync.

AddThis
AddThis for Firefox is the best add-on to make sharing and bookmarking simple. Have all your favorite web 2.0 social networking, bookmarking, blogging, and e-mail services at your fingertips. Share any page, anytime, with anyone.

Fast YouTube Download
Download any YouTube video across the web with a single click!
Supported format: FLV, High-Quality MP4 (iPod), HD (High Definition) and 3GP.

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.

Download The addons for Firefox

https://addons.mozilla.org/en-US/firefox/