Very nice list, although most of them, I already have…some of you know my, eh…extensive list of extensions…
Updated list available here
But! for the sake of security…heres the current list:
here are hundreds of firefox extensions on the web. Which ones do you use? Here is my attempt to collect the 50 best and popular firefox extensions
which make your browsing, downloading and navigation in Firefox as easy
as possible, while harnessing the full power and features of Firefox.

Extensions
are small add-ons that add new functionality to Firefox. The list has
been classified on the basis of some broad categories and most of them
work on the latest version of the Firefox Web Browser 1.5. Firefox
latest version allows easy update of these extensions than ever before
with a few clicks. You need to check that similar functioning
extensions may not work with each other. You probably use many of them,
and may find some new good extensions here…

Control Firefox Tabs

  • All-in-One Gestures
    – merges the popular following extensions for management of mouse
    gestures, scrolling and power navigation. (Mix of Mouse Gestures,
    Rocker navigation, Tab scroller, History scroller, Link tooltip and
    Autoscrolling extensions)
  • Tabbrowser Preferences
    – a comprehensive UI for changing a number of the hidden tabbed
    browsing preferences in Firefox. It also provides the ability to
    control how internal and external links are opened in the browser and
    how the browser will react when links are sent to it.
  • Tab Mix Plus
    – More tweaks added to tabs. Ability to select and open muliple links
    in tabs, open link in a duplicated tab, merge tabs and close tabs from
    similar domain…
  • Undoclosetab – Allows accidently closed recent tabs to be reopened.
  • Tab X – Adds a small close button to each of the browser tabs, and removes the close button from the end of the tab bar.
  • Restart Firefox
    – Adds “Restart Firefox” menuitem to “File”, and a toolbar button for
    easy restarting. Great for when you install themes or extensions and
    need to restart Firefox.
  • Sessionsaver – restores the pages you had open the last time you used Firefox – even if it crashes or there is a powerout.

Control Website Links

  • Linky
    – Lets you open or download all or selected links, image links and even
    web addresses found in the text in separate or different tabs or
    windows.
  • WebMailCompose – Makes mailto: links load your webmail’s compose page and adds a Compose link to the context menu.
  • Linkification – Allows Firefox (0.9+) to view plain-text URLs and e-mail addresses as actual links
  • IE View – allows the current page or a selected link to be opened in Internet Explorer. Very useful for those IE only pages.
  • Paste and Go
    – lets you paste an URL from the clipboard into the address bar and
    load it as a single step, either via the adress bar’s context menu or
    by pressing Ctrl-Shift-V
  • SuperDrag&Go
    – Drag a link or picture and throw it anywehere on your webpage
    (content area) and open it on a new tab. (Not updated for FF1.5)

Control File Downloads

  • FlashGot – handles single and massive downloads with several external Download Managers.
  • PDF Download
    – Every time you click on a link, checks if the target is a pdf file
    and in this case let you choose what you want to do (open pdf file
    inside a new tab, download it to the filesystem or view it as HTML).
  • ScrapBook – helps you to save Web pages and easily manage collections.
  • DownThemAll!
    – adds new advanced downloading capabilities to your browser. It lets
    you download in just one click all the links or images contained in a
    webpage or refine your preferences using fully customizable filters.
  • TargetAlert
    – provides visual cues for the destinations of hyperlinks. If a
    hyperlink points to a something that is not a web page (in cases of
    pdf, doc, zip files etc.), then TargetAlert will try to append an icon
    to the hyperlink that represents its destination
  • Download Manager Tweak
    – modifies the default appearance of the firefox download manager and
    allows it to be opened in a separate window, the sidebar, or a tab.
  • Download Statusbar – is a browser extension that allows you to keep track of ongoing and completed downloads in a hide-away statusbar
  • Disable Targets For Downloads – Prevents sites spawning blank windows when clicking binary downloads.
  • FireFTP – is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.

Control Webpage Behaviour & Navigation

  • GreaseMonkey – Install user scripts and change the behavior of any web page
  • Image Zoom
    – Right click on an image and select a zoom option from the popup menu,
    or, hold down the right mouse button in combination with the mouse
    wheel to zoom in or out on an image.
  • Fasterfox
    – Speed up Firefox. Dynamic speed increases can be obtained with
    Fasterfox’s unique prefetching mechanism, which recycles idle bandwidth
    by silently loading and caching all of the links on the page you are
    browsing. Also tweaks many network and rendering settings such as
    simultaneous connections, pipelining, cache, DNS cache, and initial
    paint delay.
  • SpellBound
    – enables spell checking in web forms such as html textarea / input
    elements (html input password elements are not checked by SpellBound)
    and rich text form elements. This allows you to spell check forms
    before submitting them.
  • BugMeNot
    – Bypasses compulsory web registration using the BugMeNot without the
    hassle of surfing to it and querying its database everytime.
  • Copy Plain Text
    – Copies text without formatting. Have you ever copied something and
    been annoyed that the text formatting (bold, font size, etc) came with
    it? Don’t you wish you could just copy the text itself, without having
    to copy it, paste it into notepad, then copy it again?
  • Google Images Re-Linker
    – This will let you click the thumbnail images on images.google.com,
    skip the referred framed page, and jump straight to the full-size image.
  • Stop-or-Reload Button – Makes the Stop and Reload button behave like a single one (as in Safari).
  • Extended Statusbar
    – adds an Opera-like statusbar for Firefox that shows number of loaded
    images, bytes downloaded, average download speed, load time and
    percentage of the page loaded.
  • Resizeable Textarea – Resize small textareas in forums to your needed size avoiding scrolling.
  • Adblock – allows the user to specify filters, which remove unwanted content based on the source-address. Block ads, applets, flash, embedded-media etc.
  • Flashblock – blocks all Flash content from loading on a webpage.
  • Sage – add a lightweight RSS and Atom feed aggregator which integrates with Firefox’s bookmark storage and Live Bookmarks.

Control Privacy / Security

  • SwitchProxy
    – lets you manage and switch between multiple proxy configurations
    quickly and easily. You can also use it as an anonymizer to protect
    your computer from prying eyes.
  • NoScript
    – allows JavaScript, Java (and other plugins) only for trusted domains
    of your choice. This whitelist based pre-emptive blocking approach
    prevents exploitation of security vulnerabilities with no loss of
    functionality
  • Always Remember Password
    – Instructs web sites to always remember your password. Some sites like
    Yahoo Mail, Hotmail, and banking sites instruct the browser to never
    allow your password manager to retain your information.

Control Web Searches

  • Google Toolbar for Firefox
    – Lets you search google and all its services easily. Also powered by
    Google Suggest (Get query suggestions as you type in the search box),
    SpellCheck, AutoFill, Pagerank of webpage, access to gmail,
    WordTranslator etc.
  • CustomizeGoogle
    – enhances Google search results by adding extra information (like
    links to Yahoo, Ask Jeeves, MSN etc) and removing unwanted information
    (like ads and spam).
  • NextPlease!
    – allows you to assign keyboard shortcuts to jump to next and previous
    links on search results pages, like Google, Yahoo, Ebay, Amazon, and
    many other sites.
  • BetterSearch
    – enhances Google, MSN Search, Yahoo Search, A9, Answers.com,
    AllTheWeb, Dogpile.com, del.icio.us and Simpy.com bookmarks by adding
    previews (thumbnails) and Amazon product images and info etc.

Control Your Music / Video

  • FoxyTunes – Listen to music while surfing the Web, and control your favorite media player without ever leaving the browser.
  • MediaPlayerConnectivity – Allow you to launch embed video of website in an external application with a simple click

Control Your Email

  • Gmail Manager –
    Allows you to manage multiple Gmail accounts and receive new mail
    notifications. Displays your account details including unread messages,
    saved drafts, spam messages, labels with new mail, space used, and new
    mail snippets.
  • Gmail Notifier – checks and notifies your email in Gmail accounts.
  • Gmail Delete Button – Add a “Delete” button to Gmail’s interface.
  • AJAX Yahoo! Mail – Preview your messages with a single click. Adds AJAX support to the Yahoo! mail.

Control Your Bookmarks

  • Bookmark with Del.icio.us
    – Integrate del.icio.us right into Firefox. Easily access your
    bookmarks and post new bookmarks. Post a bookmark without leaving the
    page you are on.
  • Bookmarks Synchronizer
    – lets you connect to an FTP server and synchronize your bookmarks that
    are stored in an XML file. Set if to automatically download the file on
    startup or upload it when you close your browser.
  • StumbleUpon – is an intelligent browsing tool for sharing and discovering great websites.

Control Other Popular Services

  • Adsense Notifier – Displays your Adsense earnings on the statusbar.
  • Forecastfox – Get international weather forecasts from AccuWeather.com, and display it in any toolbar or statusbar
  • StockTicker – Shows your favorite stocks in a customizable ticker.
  • DictionarySearch – Looks up a user selected word in an online dictionary you selected.
  • Web Developer – Adds a menu and a toolbar with various essential web developer tools.

Happy Extension Catching!