Adblock Plus - Tired with ads ? then go for this Firefox add-on. Adblock Plus enables you to block ads.If you wish to enable ads for a particular site then you can create an exception.The only drawback of Adblock Plus is the exception filter creation needs a little knowledge.(Example exception filter - add @@|http://nextdoornerd.blogspot.com to add an exception for this particular site,remember you have to do this manually). You can learn to write Adblock Plus filters from http://adblockplus.org/en/filters. It is a nice how-to article on how to write custom filters.
Click here to get DownThemAll
StumbleUpon - Go check it out, a cool add-on helps you discover Web sites based on your interests.This is a useful site when you are bored and looking for something to browse or when you want something funny or informative to browse through.
PicLens - It is an awesome add-on that transforms your Firefox into a next generation Image and Video viewer with their stunning new Image wall.(hey if you have Google desktop installed then you can use PicLense to show your offline Images to your friends,and see them say WOW!)
PrintPDF - Installs a driver printer on your machine and lets you print your web pages to pdf files (Mac users, you already have this feature!).
GSpace - This add-on allows you to use your Gmail as an online storage space.So now you can upload files from your hard disk and access them from anywhere.
Get GSpace from here
ChatZilla - Love your old IRC chat ? , then here is an add-on.ChatZilla - An easy to use IRC client add-on for your Firefox.Now you can chat without leaving Firefox.
PDF Download - Did you ever think of downloading web pages as PDF files ?. Here is a very useful add-on that help you download web pages as PDF files.
Download PDF Download
FoxyTunes - FoxyTunes lets you control almost any media player and find lyrics, covers, videos, bios and much more with a click right from your browser.Download FoxyTunes
Sage - Don't want to leave your browser for reading your favorite rss feeds ? here is a handy plugin with some cool features and easy to use interface for handling feeds.
Zotero - Easy-to-use Firefox extension to help you collect, manage, and cite your research sources.Download Zotero
[this post will be updated frequently with more better plugins as I come across them.. :) so don' forget to check this out]