Extensions are small add-ons that add new functionality to Firefox. They can add anything from a toolbar button to a completely new feature. They allow the browser to be customized to fit the personal needs of each user if they need additional features, while keeping Firefox small to download.
There are over 1000 extensions available for Firefox. This may make it difficult for people to find what they want. The links on this page are intended to help while also remaining current. As such they will not generally point to specific extension but rather where to look for them. To look at links by category this page is a good start. Additionally the headings below provide a link to mozilla.org pointing to the specific category - this means that the information will always be up to date.
A good place to start looking for extensions is here for recommended extensions.
The most popular extensions are to be found here.
There are many themes available at Mozilla Update which make Firefox more attractive. The most popular ones are here.
In addition, Mozilla has recently launched Fashion Your Firefox, a service for users who don't want to go through the entire library of addons to choose just a few good ones based on what they use Firefox for.
- EditCSS- Stylesheet modifier in the Sidebar. Allows you to edit any site's CSS file on the fly.
- Web Developer Extension- Must have extension for anyone dabbling in web design.
- Vista-aero — A theme for Firefox meant to look like Internet Explorer 7; with Aero-like visual effects
- BBCode — Enables you to quickly and easily insert various BBCode tags in editable textboxes from the context menu. Very handy for customizing posts on forums.
- Calculator — A simple calculator extension.
- Calculator — An advanced calculator extension.
- Calendar — An iCal compatible Calendar for Firefox. You can download calendars for your favourite sports teams, etc., from iCal Share
- ChatZilla — A simple, straightforward Internet Relay Chat (IRC) client.
- Cookiepie — An extension to handle multiple web accounts simultaneously, opening many Gmail, Yahoo, Hotmail accounts in different tabs on the same browser.
- FoxyTunes — Adds media player controls within Firefox. Currently supports many different music players across multiple platforms.
- Gmail Notifier — A Gmail notifier that integrates into the browser's user interface.
- FasterFox — A simple-to-use performance and network tweaker. Fasterfox allows you to change certain setting to make Firefox load pages more quickly, depending on your computer speed and Internet connection.
- Quick Note — A note taking extension with advanced features.
- StumbleUpon — Lets you 'stumble upon' websites that have been recommended ("thumbed up") by friends and community members with interests similar to your own. See also StumbleUpon website.
- WebmailCompose — Makes mailto: (email) links load a webmail mail compose window and adds a WebMailCompose link to the context menu. Now supports selected addresses.
- All-in-one mouse gestures — Enables you to use the mouse for a wide variety of customizable shortcuts.
- Linkification — Injects CSS on the fly making text URLs clickable with the default option to suppress referrer information.
- Paste & Go — Lets you paste a URL from the clipboard and directly load it.
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- Adblock Plus — Blocks or hides advertisements and other annoyances from web pages, while still retaining the correct page format. It is designed to allow you to block flash and image banners built into the web page, as well as scripts. You add your own filters using wildcards (*) for simple filters, or RegEx (regular expressions) for advanced ones. An excellent maintained set of filters is here.
- Flashblock — Allows you to easily block flash animations. Flash animations are replaced with an icon you click to play. May conflict with Adblock Plus and crash on some flash-intensive sites.
- Image Zoom — Adds zoom functionality for images. Functionality is also included in the Mouse Gestures extension.
- termBlaster — Lets you search selected text using search engines selected by context menu. It comes with 110+ search engines from encyclopedias to translators to web-search engines organized in folders.
- Dictionary Search — Looks up selected word in a (customizable) online dictionary. Functionality is also included in the SmartSearch extension, which lacks the 4 search limitation.
- Googlebar — The Google Toolbar for Firefox. Additionally, Googlebar Lite provides a simpler interface (which is recommended for new users). 
- Groowe Search Toolbar — Groowe Search Toolbar bundles Google toolbar, Yahoo, Ask Jeeves, Teoma, Amazon, Download.com and others. The application is available for Internet Explorer as well.
Tabs and windows
- Focus Last Selected Tab — Brings focus to the last selected tab when closing the active tab. Functionality is also included in the TabMix extension.
- Session Saver — Saves the open tabs when closing the browser, and restores them upon restarting it. Functionality is also included in the TabMix extension.
- Single Window — A simple extension that allows Mozilla to fully utilize the built-in tabbed browsing behavior. Traps links that would normally open in a new window. Functionality is also included in the TabMix extension.
- Tabbrowser Preferences — Enables enhanced control for tabbed browsing. Most functionality is also included in the TabMix extension. Recommended for users who would like to change the way tabs behave in Firefox.
- TabMixPlus An extension that allows considerable customisation in the use of tabs and windows as well as a "restore" function that is as useful (or better) then the built in FireFox one.
- Undoclosetab — Reopen a closed tab. Functionality is also included in the TabMix, SessionSaver, and Mouse Gestures extensions.