I've been using Mozilla Firefox for over 2 years now, and over the course of this time, I've gathered a list of the most useful extensions I've been using since I begun. These plug-ins might not be the best, nor the most useful, but they are my favorites and thought I might introduce them to you.

1) DownThem ALL!

3) Download Helper

DownloadHelper allows you to download embedded content from websites, but is most useful for downloading videos off Youtube and many of its clones. It will search the page you are browsing to look for embedded files, namely video files. If it finds something, its icon will colourize (it's black and white when nothing is found). You just need to click on it, and you will have a list of downloadable content. I've not been able to make it work with DTA though. That would have been great. You can still download normally, copy the link, then put it in DTA.

4) Tabmix Plus

This plug-in is very famous, and adds many new and innovative features to Firefox's already powerful tabbing features. You will get such features as protecting tabs from accidental closing, re-open closed tabs, size your tabs, colourize them, put a progress bar on them, and define what happens when you click, right-click and middle-click on the tabs or tab bar. It's difficult to explain the extent of its features, so go check it out. In short, it allows you to fully customize the way tabbing works in Firefox. Another useful function: It allows you to drag text links from the main window to the tab-bar for opening.

5) Session Saver

If ever you have wanted to save a number of opened windows or tabs for later viewing, or even to load them at every starting of Firefox, this plug-in is for you. Sometimes, I need to save a group of tabs which I find interesting, and want to open them later. Session Saver allows you to save your session and re-open it later on. You can also define a session consisting of tabs and windows, and you can have that session restored every time you start Firefox. Eg. You can have Firefox load your favourite blogs at start up.

6) Brief

Brief is very useful if you like to read RSS and other kinds of news feeds. It's an all-in-one reader that manages your feeds, refreshes them and displays notifications when there are updates. You can have it open in a new tab, and it's all built in. The left part shows your feeds, while the right part shows the contents. It has many options such as the useful "mark as read on viewing" feature. I like Brief over other Feed readers because its features are grouped in one tab, and clicking on a feed opens it in Brief's tab, not other new tabs so it doesn't interfere with my browsing.

7) Greasemonkey

Greasemonkey is quite an innovative plug-in that allows you to add scripts that in turn add new functions to sites. For example, there are numerous scripts that allows you to "clean-up" a site to remove content such as ads and animations, making the site more user-friendly and easy to use. It may also add other functions such as direct links to parts of the site. Basically, it allows you to execute custom script/codes to give you new functions for your sites. You can find many scripts at UserScripts.org ready for use.

8) Bookmark Duplicate Detector (Bookmark DD)

As its name suggests, Bookmark DD warns you about adding bookmarks as duplicate. It checks your existing bookmarks, and if the one you are currently adding already exists, it warns you and give you its existing location. This plug-in has allowed me to regain considerable order in my bookmarks. You can also have it scan your existing bookmarks to check for duplicates. It warns you on adding new bookmarks automatically, if ever it finds an existing match. It can also help you backup your Bookmarks file.

9) FEBE

A very useful plug-in that allows you to backup Firefox almost as a whole. It backups your plug-ins, themes, bookmarks, settings and other stuff, and puts them in a folder of your choice. Then, if ever you have to re-install Firefox on a new PC, you can just bring that folder with you, install FEBE there first, then have it restore the backup. After a restart of Firefox, you will have your old settings, bookmarks and everything else. It can also automatically backup your stuff, and you can choose what you want to backup, and where you want the backup to be stored, and how it should be named.

10) Linkification

Linkification has a simple use. It makes text links clickable. For example: http://www.google.com. If you had Linkification installed, that google link would be clickable. You'd no longer have to use copy-paste to address bar to go there.

11) Snap Links

Initially, this article would have contained only 10 plug-ins, but while finding links, I came across this one. Snap Links allows you to draw a rectangle around multiple links, and open them in new tabs, new windows, bookmark them, download them and a number of other operations. Very useful when you come up with a site with multiple links and you want to open all of them without clicking on each of them. Eg. Here itself, you could have opened links to all of the 11 plug-ins at once if you had Snap Links installed.

That's all for now folks. Try checking out these plug-ins. They are very useful. But remember, abuse of Firefox plug-ins will slow-down your browser and greatly increase its memory usage. So use plug-ins moderately! :P