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There is nothing like firefox, when it comes to the customization of your browsing experience. There are  thousands of addons available to choose from. I have prepared a list of 23 addons that are must for firefox 3.0 users. I have divided them into 5 catogories according to their use which are multimedia, security, comfortness, professional, and other important addons.
1. Comfortness
The addons that I have mentioned under this category will surely make your browsing experience more comfortable.
a) All in one sidebar
All-in-one sidebar is a firefox add-on that you must have if you are using firefox 3. It is a cool sidebar which allows you to mangae your on going downloads, bookmarks, extensions and themes etc all at one place.You can also view different kinds of informations of the current web page like general information, permissions and media informations. You can also see your browsing history in the sidebar. Heres a screenshot of the sidebar.You can also click to toggle the sidebar automatically.
b) DownThemAll
This one can prove to be very useful in many ways. For example, if you are downloading something and the web connection is lost then all files will be lost too with the default download manager of firefox. With DownThemAll you can recover your downloads that were stopped abruptly, it also helps in accelrating the download process.
c) Interclue
Interclue tells you that  what is behind the link before you click it.  Small icons appear on mouseover to warn you about dead links, Malware or Phishing Sites, etc, and for any valid links Interclue provides compact link previews, relevant extra information and useful next action buttons. In simple words when you hover your cursor over any link, a small icon will appear next to it, and when you hover over that icon it opens a pop up window that shows a preview of what that link leads to.
d) IE Tab
Sometimes there are some websites that are not displayed properly in firefox because they are written exclusively for internet explorer. Mostly people will open that site in IE separate window, but IE tab has an answer to it. With IE tab you can open a web page in IE by right clicking on the link and select the option ‘Open Link In IE Tab’.
2. Security
Firefox is probably the only browser that is most prone to security risks and even more secure to combat those risks. To ensure security, firefox developers regularly patch loopholes if any. Therefore use of addons that ensure security is must.
a) FoxyProxy
FoxProxy hides your IP address. Generally when we browse the net, we leave traces and in some cases these traces are used for hacking purposes which can be anything like your mail id, bank account number etc. In such cases FoxyProxy is extremely useful. It uses Tor and the Privoxy global router networks for enycrption . Animated icons show you when a proxy is in use. Advanced logging shows you which proxies were used and when they where used.
b) NoScript
It probably one of the best securty addons you can get. It allows JavaScript, Java and other executable content to run only from trusted domains of your choice, e.g. your home-banking web site, and guards the “trust boundaries” against cross-site scripting attacks. It is also the winner of the 2006 PC World World Class Award.
c) Secure Login
Secure Login actually tightens the security. It provides a phishing filter that compares the second level domains and warns you in the case of websites with fake credentials. It also prevents malicious JavaScript code to automatically steal your login data. It Integrates with Mozilla Firefox password manager.
d) Adblock Plus
No website will really want its adds to be blocked but some sites feeds you with overdoze of adds which should be blocked. Right-click on a banner and choose “Adblock” from the context menu – the banner won’t be downloaded again. The addon uses various filters for blocking purpose.  With adblock you can block images, flash movies and many other advertisements.
3. Multimedia
Under this category I have chosen the most appropriate addons according to me that will manage multimedia content more efficiently.
a) Gspace
Gspace is one of the best firefox addons available today. Gspace 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. You can also partition the space.
b) Video Download Helper
Another great addon which allows you to download videos from almost every site, even from the site that doesnt allow downloads. The automatic search in it is also a very cool feature which can provide you with a list of hundereds of video portals. This works also for audio and picture galleries. Just surf the Web as you are used to, when DownloadHelper detects it can do something for you, the icon gets animated and a menu allows you to download files by simply clicking an item.
Download Video Download Helper
c) PicLens
It is the most stylish addon that firefox has. This addon shows the images from the databases of search engines and portals in a super stylish 3d view and control slideshows in full screen mode. It transforms your browser into a visually stunning experience for searching, viewing, and sharing online photos and videos. The grid is often called as “3DWall”. You can watch the latest news, sports, and entertainment feeds by clicking the “Discover” button.
d) FireShot
If you want to use the graphical content of any web page for your work, FireShot can help you by taking the screen shots. It is also possible to choose whether entire web page or only visible part of this page should be captured. You can also add annotations and upload to server, save (PNG, GIF, JPEG, BMP), print (NEW!) copy to clipboard, send to external editor or e-mail it.
e) FoxyTunes
FoxyTunes is a very flexible tool to use. FoxyTunes lets you control almost any media player and find lyrics, covers, videos, bios and much more with a click right from your browser. It supports WinAmp, iTunes, Yahoo Music Engine, Pandora, foobar2000, Windows Media Player, Xbox Media Center, Musicmatch, Quintessential, J. River, jetAudio, XMPlay, MediaMonkey, Media Player Classic, Sonique, wxMusik, Real Player, XMMS, Noatun, Juk, Amarok, Music Player Daemon, Rhythmbox and many other players.
4. Professional
Well this category contains addons for professional users. They can prove to be very helpful for them.
a) Greasemonkey
You can re-design any website according to you with a little bit of javascript. The best thing in it is that you can write your own scripts too, but you need to have some technical knowledge for it. With the help of greasemonkey it is possible to display particular topics on a heavily loaded web page.
b) Stylish
Stylish allows you to edit the look of any website with the help of CSS. Stylish is to CSS what Greasemonkey is to JavaScript, and unlike other methods of using user styles, most styles take effect immediately. Readymade scripts are also available, which can be downloaded from the manufacturers website.
c) Firebug
Firebug is an analyser, which allows you to monitor, debug  CSS, HTML, and JavaScript live in any web page. Firebug also lists some hidden processes as well.
d) SQLite Manager
It allows you to manage any SQLite database on your computer. An intuitive hierarchical tree showing database objects. Helpful dialogs to manage tables, indexes, views and triggers. You can browse and search the tables, as well as add, edit and delete the records. There is facility to execute any sql query.
e) Web Developer
This a must have for any geek out there. It adds a menu and a toolbar with various web developer tools. The toolbar has all the tools required for testing by professionals. You can deactivate java, formfields and even javascript with it. Inexperienced users can also try hands on it which can be useful for them.
f) FireFTP
FireFTP is a functional FTP client which makes the task of managing servers and websites a lot easier. Downloading data also becomes more easy with it.
5. Other Important Addons
a) BlogRovR
If you store websites sorted according to topics of your choice, it will fetch all the posts, news and comments from them. It also suggests the popular items. RovR’s resizable tray slides in to show you summaries of posts about the current page. Click on a summary to read the full post, right on the page it’s about, or CTRL-click to open a new tab on the blog post.
b) Locator
Locator allows you to find addresses’. Select the address you want to find, right click on it and choose locate on Google Map.
c) SamePlace
SamePlace is the best instant messenger. Other than chatting it also allows you to share pictures and play games.
It not only allows you to make schedules in the browser, but also you can coordinate them with friends . ReminderFox remembers things so you don’t have to. ReminderFox makes sure you remember all of your important dates via easy-to-use lists, alerts, and alarm notifications.
If you know some more essential firefox addons that I have missed, you can discuss in comments below.
http://labs.mozilla.com/projects/ubiquity/
Although Ubiquity is still in the making, it works, and it works well. I definitely recommend it to you.