Recommended Firefox Extensions

1. Tab Mix Plus

Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. I strongly recommend the options of Select the tab pointed (Tool->Tab Mix Plus Option->Mouse->Mouse Gestures). It greatly decrease the amount of clicks when browsing.

2. Session Manager

Session Manager saves and restores the state of all windows - either when you want it or automatically at startup and after crashes. For example, if you start you work everyday by opening some webpages , you can store them in a session. Additionally it offers you to reopen (accidentally) closed windows and tabs. If you're afraid of losing data while browsing - this extension allows you to relax...

3. Firefox Showcase

If you habitually find yourself awash in open tabs (It seems that one of my classmates usually runs into such situation), clicking around looking for the page you need, Firefox Showcase will save you a lot of aggravation. Once you install the extension, you'll have a new Showcase submenu under the View menu. From here you can choose to show thumbnails of all tabs in the current window or all tabs in all windows. Firefox has lots of options and keyboard shortcuts, however I will never dive into those complex options. Simply click F12 to get the thumbnail view, uparrow and downarrow to select the intended tab and enter to see the full view the select tab. I can also exit the thumbnail view by clicking Esc. That's all. For more, see View->Showcase and for a sidebar view, follow View->Sidebar->Showcase Sidebar.

4. Download Statusbar

If you're tired with that sometimes-pesky Downloads window that pops up whenever you download a file in Firefox. Download Statusbar suppresses that window from popping up, and instead provides you the same information in the status bar at the bottom of the browser window. You can roll your mouse over the filename and get a pop-up tool tip with some extra information about your download, too

5. DownThemAll

DownThemAll is is download manager and accelerator. It lets you download all the links or images contained in a webpage and much more: you can refine your downloads by fully customizable criteria to get only what you really want! Simply, it saves you them time to open a shell and use wget.

6. Fotofox

Using Fotofox, you are able to grab a picture from any pages to the Fotofox sidebar, title it, tag it and upload them to your flickr album with a simple click. It is also compitable with other picture hosting site such as Tabblo, 23hq, Smugmug, Marela, and Kodak EasyShare Gallery.

7. Fasterfox

Exactly I don't know its performance.

8. Google Browser Sync

Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings – including bookmarks, history, persistent cookies, and saved passwords when you are using firefox on several computers. It also allows you to restore open tabs and windows across different machines and browser sessions. Alternatively You can choose to sync cookies, or not to sync cookies, but you can't make the decision based on individual cookies. Suppose that you are now using a public computer. At first you can install this extension, sign in with your google account. The Google Browser Sync begins to synchronize your firefox settings on the server and thus you get a familiar firefox in the public computer. When you are going to leave, choose Tools->Google Browser Sync->Stop Syncing and click Tools->Clear Private Data... (Ctrl+Shilt+Delete) to clean your personal leaved on the public computer.

9. Google Notebook

In my opinion Google Notebook is a simple but great product though most people ignorate it. This is possibly because that there is not a convinient on-one-click client interface and that the usability is poor in the webpage user interface. For example, I have been long hoping for the tag feature and sharing ability based on a note. Google Notebook it the client interface. It simple the process you make notes. Just select any part of the webpages, whether be it text or picture, click Note This from pop menu activated by right mouse button. I believe Google notebook will outrun Clipmarks or other similar services.

10. Firefox Google Bookmarks

Firefox Google Bookmarks (GBookmarks) creates a menu to access your google bookmarks from any computer. ( Your google bookmarks resides in your search history). Additionally it server a backup mechanism for all bookmarks. This extension may overlap with the Google Browser Sync. Personally, I consider the Bookmark in firefox a lightweight bookmarks that store only the everyday used links and use GBookmarks as a heavyweight repository of all links that may be useful some day.

11. StumbleUpon

StumbleUpon is also a bookmark service that incorporate social network elements. It resembles delicious. StumbleUpon lets you "channelsurf" the best-reviewed sites on the web. It is a collaborative surfing tool for browsing, reviewing and sharing great sites with like-minded people. This helps you find interesting webpages you wouldn't think to search for. You can also share pages of interest within a community. Particularly, it will insert into your google search results which shows you other people's rating and reviews of the search result.

12. Greasemonkey

Greasemonkey basically allows you to add JavaScript to any Web page, which implies infinite control over the behavior of web pages. Greasemonkey is not for the faint of heart. The good news is that there are many generous souls out there who share the scripts they create. Check out userscripts.org for a script repository. If you want to write your own scripts, try diveintogreasemonkey.org or pick up Mark Pilgrim's Greasemonkey Hacks from O'Reilly Media. Personally I strongly recommend Gmail Macros. After installation, it empower gmail with striking and convinient keyboard shortcuts like Google Reader.

13. Adblock

Adblock is a content filtering plug-in for the Mozilla and Firefox browsers. It allows the user to specify filters, which remove unwanted content based on the source-address. Adblock supports two types of filters: simple, and Regular Expression. Adblock will also provide a default list of filters, which is enough for a lazy person like me.

14. ScrapBook

ScrapBook is a Firefox extension, which helps you to save Web pages and easily manage collections. This enable you to surf web pages off line. You can also directly copy the ScrapBook folder from your firefox option folder to other computers and see what you saved there.

References:
20 must-have Firefox extensions
Firefox Add-ons Recommended Add-ons
千种风情千种树: My Firefox Extensions