Recommended Firefox add-ons (extensions)

Summary: After installing Firefox, you may want to add one of the following extensions (this list is being continuously updated).

Disclaimer: Before installing (or enabling) a new Firefox add-on, it may make sense to check if it belongs to the list of problematic extensions, as well Mozilla's own Add-ons Blocklist.

Here is the list of the Firefox add-ons that I use and can recommend to others:

Aardvark can be used for cleaning up a page prior to printing it (by removing unwanted elements, such as advertisements), for making the page more readable, and for analyzing the structure of a page.
Adblock Plus gets rid of ads and banners that often take longer to download than everything else on the page. It works in conjunction with the Element Hiding Helper add-on. The Adblock Filterset.G Updater add-on is also recommended for use with Adblock Plus, but I had to disable it some time ago because it tried to perform a run-time check against an offline site significantly degrading my browser performance (the problem may have been fixed since then, I do not know).
All-in-One Sidebar lets you quickly switch between sidebar panels, view dialog windows (downloads, extensions, and more) and websites sources in the sidebar.
Answers provides facts and definitions from over 100 dictionaries, encyclopedias and almanacs without having to search through lists of search engine links. Just point at any word, hold the ALT key (CTRL in Linux) and click. Upon letting go, an AnswerTip in the form of a pop-up "information bubble" appears on the screen explaining the term.
Auto Copy adds options to context menu allowing to open a text URL (which is not marked as a hyperlink in HTML) directly in the browser without having to copy and paste it.
Clear Cache Button adds the Clear Cache button to the toolbar customization panel. Open this dialog to find the clear cache button and then drag it onto the toolbar. A similar extension is available for IE.
Clipmarks allows you to easily share parts of web pages (text, images and video), which is handy if you want to send someone a quote from a web page and not the entire page.
Configuration Mania displays advanced Firefox settings for editing.
CookieSwap lets you quickly switch between multiple web accounts (like Gmail) without having to log off.
Cooliris Previews displays contents of the web page referenced by a hyperlink in a preview window (simply mouse over the Cooliris link icon, and a preview window will instantly appear to show you the underlying content without leaving your current page). In addition, it e-mails links instantly, searches Google, Wikipedia, or a dictionary for highlighted words or phrases, and offers other minor conveniences.
Coupon Craze - Coupon Notifier shows coupons and discounts offered various online merchants when you access their sites.
Copy Plain Text gives you an option to copy text without the formatting. You can even set it to trim extra space in and around the copied text.
CyberSearch Adds advanced Google search capabilities to the Firefox 3 address bar.
Dictionaries work with the spell-checking feature in Firefox. Select the one that is appropriate for your language and locale, such as en-us for English speakers residing in the United States.
Defnr offers a way to quickly get word definitions, including synonyms.
Dictionary Tooltip displays the meaning of a selected word in a small pop-up window. To invoke, just press CTRL+SHIFT+D, or double-click the word, or right-click the selected word.
DownThemAll! lets you download multiple targets (pictures, audio files, etc) referenced on a web page.
Duplicate Tab allows you to clone a tab along with its history.
Extension List Dumper lets you generate a list of installed Firefox extensions and themes.
FireShot creates screenshots of web pages. It also provides a set of editing and annotation toolsallowing to insert text and graphical annotations, which is especially useful for web designers, testers and content reviewers. It's possible to choose whether entire web page or only visible part of this page should be captured. Screenshots can be saved to disk using various graphics formats, copied to clipboard, e-mailed, and sent to external editor for further processing.
Firefox Universal Uploader (FireUploader) provides a single and intuitive interface for uploading files to multiple sites including Flickr, Picasa, Youtube, Box.net, and Facebook. In most cases, FireUploader works better than the native uploading tools.
Flashblock blocks all Macromedia Flash content (such as YouTube movie clips) from loading; it leaves placeholders on the web page that allow you to click to download and then view the Flash content.
Form Saver saves form field data as auto-fill bookmarklets. Works with inputs, check boxes, radio groups, text areas, and list boxes.
Foxmarks Bookmark Synchronizer keeps your bookmarks, passwords, toolbar buttons, persistent cookies, history, and other settings in sync between different computers.
FoxyProxy offers a better proxy support than the built-in capability.
Freecorder can record streaming audio to an MP3 file, which is very useful for generating podcast files from podcast streams (e.g. old podcasts which are not available for downloads).
Fresh Bargains provides a convenient way of searching for best deals published at different bargain hunting sites (such as DealCatcher, FatWallet, SlickDeals, and many others) and sending yourself alerts when a particular product appears on these sites. While Fresh Bargains does not offer an add-in, you can add it to the list of search providers (in the Search toolbar).
GDirections finds directions on Google and Yahoo Maps based on your selected text and one of various home addresses.
Google Notebook extension lets you add notes to your online notebook entries. This is really handy for doing research and saving links, references, and quotes from different web sites.
GooglePreview inserts preview images (thumbnails) of web sites and Amazon products into the Google and Yahoo search results pages.
Gspace allows you to treat your Gmail account as free online storage. With this add-on, you can transfer files between your computer and Gspace. (NOTE: Some users reported that after transferring large amounts of data, their Gmail accounts had been locked out for 24 hours; use at your own risk.)
HP Smart Web Printing allows you to print clips from multiple web pages (or elected parts of one page) as a single page. This extension works with any printer (not just HP).
HyperWords adds a context menu for highlighted words with a number of options, including multiple searches (Google, blogs, etc.), references (Wikipedia, Thesaurus, etc.), translation, and others.
IE Tab embeds Internet Explorer in a Firefox tab. Use this feature to access web site optimized for Internet Explorer (such as Microsoft) without switching to Internet Explorer.
IE View Lite can be used to open a link in Internet Explorer. Use it for the sites which do not get displayed correctly in either Firefox or IE Tab.
Image Preview Extension allow you to see a thumbnail of the CraigsList images without clicking on each ad.
ImTranslator provides convenient access to online translation service powered by PROMT translation server (PROMT Ltd.) and AltaVista. It includes the following modules: translation editor, virtual keyboard, spell-checker, dictionary, decoder, back translation, email client. With ImTranslator you can enter multilingual text, check it for correctness, adjust unreadable messages, translate in different languages, look up words, print results and send email. The add-on installs the context and tool menus, as well as a toolbar image button.
Interclue opens a small pop-up windows showing a preview of the page pointed by a hyperlink. It works similar to Cooliris Preview, but is a bit more flexible (after using it for a few minutes, I think I like Inerclue better).
Link Alert changes the cursor to indicate the target of a hyperlink. This extension will allow you to see whether a link will open in the current window (tab) or a new one.
Locator displays highlighted address in a Google Map popup.
Map+ displays a Yahoo! map of a selected address without having to open a new window or tab. After enabling Map+ you may need to adjust the size of the pop-up window displaying the map in the add-on options.
Meebo installs a sidebar which you can use for instant messaging (IM) via the browser. Supports different IM protocols (networks) including AIM, MSN, Yahoo!, GTalk, ICQ, and Jabber. This extension allow using IM across corporate firewalls.
Mini Map Sidebar allows you to drag-and-drop addresses to generate maps and driving directions. It provides a whole range of features for working with addresses.
Minute Minder for Cingular Service displays your remaining Cingular minutes or text messages in the status bar. It also displays a detailed account overview when you hover your mouse over the extension in the status bar. Minute Minder works with both individual and family plans. For more information check out the extension's blog. [Update: This add-on has been renamed to Minute Minder for AT&T Service, a new version of which could be downloaded from the author's web site; however, after AT&T redesigned its web site, Minute Minder does not seem to work. For the latest information, check the news at the author's blog.]
MrUptime comes handy when you need to access a web site which is down. MrUptime adds can add such web site to the watch list and alert you when it comes back online.
MR Tech Local Install provides the tools needed to install and manage extensions and themes.
Open IT Online lets you open and edit documents and images online without the need of any software to be installed.
Ordered Preference Import/Export allows you to import and export your installed extension preferences. This is useful when installing extensions in a new profile, or synchronizing multiple Firefox installations.
PicLens allows you to view full screen slideshows of photo galleries from Google, Flickr, Picasa, Facebook, Yahoo, and a few other image services.
Picnik adds a menu option to send an image or screen shot for editing in Picnik.
PlainOldFavorites allows you to use the Internet Explorer and Windows Favorites directly from Firefox without needing to import or synchronize the bookmarks first. [Update: I don't need this extension any more.]
Pluribo: Instant Summaries scans all of the reviews on any Amazon web page and summarizes them into a single paragraph.
PriceDrop will notify you about price drops for selected items at Amazon.com, giving you a chance to take advantage of the 30-day refund guarantee.
Pronounce can pronounce English words.
Read It Later allows you to bookmark pages which you would want to read later and later retrieve them from the 'staging area'.
ReloadEvery automatically refreshes a web page displayed in the browser every so many seconds or minutes.
Remember The Milk for GMail allows you to manage to-do list in GMail. Assuming that you keep your to-do list at Remember The Milk, this extension will let you complete, postpone, and edit tasks, add new tasks, connect tasks with your emails, contacts, and Google Calendar events, automatically add tasks for starred messages or specific labels, and much more.
RetailMeNot automatically displays coupon codes for the current web site being viewed using the information retrieved from the coupon sharing web site retailmenot.com.
SavvyCircle will notify you whenever an item you mark at a selected online store goes on sale. Free registration is required.
ScrapBook lets you save and organize bits of web pages along with notes and original URLs. Good extension for research.
SearchWith adds a context menu (shown on right-mouse click) to search web for a highlighted text.
Sxipper can manage your passwords and automatically fill registration, sign-up, and sign-in forms.
ThumbStrips creates a filmstrip of thumbnails for the pages in your browser history making it easier to find the web sites you recently visited.
TinyUrl Creator adds a context menu entry to create a tiny URL for the current page, which eliminates the need to go to the TinyURL site.
UnPlug lets you save video and audio embedded on a web page. It works with many video sharing sites, such as YouTube, Google Video, and others.
VideoDownloader add a small icon on the status bar at the bottom of your Firefox window and a toolbar button, which allow you to download videos from YouTube, Google, Metacafe, iFilm, Dailymotion, and a growing list of more than 60 video sites.
WOT (WebOfTrust) warns users about risky websites that try to scam visitors, deliver malware, or send spam.
Woot Watcher monitors the Woot! web site, and keeps you updated with the current item, price, and if it is sold out. During Woot-Offs, it will also let you know what percent of the item is remaining. It can also automatically open a new browser tab or play a sound when the Woot! item changes. [If you're a total Woot! junkie, try the Windows desktop WootAgent.]

The following add-ons do not offer much value for general population, but they are indispensable for web developers:

Compact Library Extension Organizer (CLEO) works with FEBE to package any number of extensions/themes into a single, installable .xpi file. You may have a list of your favorite extensions/themes that you would like to share with others. With CLEO, you can gather them all together into a single file (called, perhaps, "My favorite extensions.xpi") and post them on a web page. Others can install the whole batch with just a couple of clicks.
Colorizer is a web development tool, which uses a color picker to change the color of HTML elements in real time.
ColorZilla gets a color reading from any point in the browser, quickly adjusts this color, and pastes it into another program. You can zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily. And there's more...
CoLT adds two menu items to the browser's context menu visible when right-clicking a hyperlink. The Copy Link Text menu item copies the hyperlink's associated text to the clipboard, while Copy Link Text and Location copies both the hyperlink's text and URL to the clipboard in the format you choose: either as plain text, as an HTML hyperlink, or as a custom format (which you may specify). A very handy tool for blogs!
CSSViewer combines style properties in all style sheets and displays the effective style applied on the selected element.
DT Whois adds button to the browser for easy access to the currently opened web site's WHOIS source.
Extension List Dumper generates a list of installed extensions, themes or plug-ins.
Fasterfox allows you to tweak many network and rendering settings such as simultaneous connections, pipelining, cache, DNS cache, and initial paint delay. Dynamic speed increases can be obtained with the unique prefetching mechanism, which recycles idle bandwidth by silently loading and caching all of the links on the page you are browsing. A popup blocker for popups initiated by Flash plug-ins is also included.
Firebug puts a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript in any web page.
Firefox Extension Backup Extension (FEBE) allows you to back up your Firefox extensions -- and (optionally) bookmarks, preferences, passwords, cookies, and just about everything else Firefox offers -- so that they can be easily synchronized between office and home computers. Backups can be performed on demand or scheduled for daily, weekly, or monthly unattended runs.
FireFTP is a very capable and easy-to-use FTP client, which supports FTP connections over proxies.
LinkChecker check links on a web page to make sure they are valid. There's also a toolbar button that can be added for easier access to LinkChecker. Just right click on your toolbar, choose "Customize" and look for the button with the underlined blue check.
MR Tech Local Install provides the tools needed to install and manage extensions and themes locally and tons of other features.
Pencil is a sketching and prototyping tool, which can be used to create mockups of screenshots.
SourceEditor allows you to view and edit HTML elements in-place. Use the button on status bar to activate/desactivate SourceEditor, then double-click (or use the context menu) to edit the HTML source of the selected element.
View Dependencies adds a tab to the Page Info window, in which it lists all files included in the current page.
View Source Chart draws a color-coded chart of a web page's source code allowing users to quickly scan and recognize the document's tags without reading a single tag, see how deeply nested an element is just by looking to its left (no scrolling/finding/reading tags), and identify an element's containing tags without having to scroll, find and read each tag.
Web Developer adds a menu and a toolbar with various web developer tools. This is a must-have extension for every web developer.

Additional references:
50 Best Firefox Extensions for Power Surfing
Firefox Extensions
Firefox Extensions cause Memory Leaks and Crashes
Reducing memory usage - Firefox
Top 10 Firefox Extensions, and More