Extensions are small add-ons that add new functionality to Firefox, but should not be confused with plug-ins.
In Firefox, add-ons is a general term that covers extensions, themes, userscripts, userstyles, service workers, and plug-ins. Extensions rely on built-in frameworks and APIs within Firefox, and are integrated into the browser after installation. Whereas plug-ins, such as Adobe Flash, use programming not native to Firefox, and are usually embedded into a web page only when said page contains code that requires Flash to be loaded.
Extensions that use the legacy XPCOM, XUL, and JetPack frameworks, are supported until Firefox 56.0. These are known both as legacy extensions, and legacy versions of Firefox, respectively. In late 2018, Mozilla ceased support for XUL-based extensions, and no longer hosts them.
While legacy extensions have been backed up by various open-source volunteers, their original installers in the form of XPI files are still available at the Internet Archive, if an extension's original address at the Mozilla Addons site is known, and if the extension page was archived before October/November 2018.
WebExtensions were initially supported in Firefox 48.0, and beginning with Firefox 57.0, only WebExtensions are supported.
What can extensions do
Extensions 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 Firefox proper remains relatively unadorned.
Many links herein will not necessarily point to a specific extension, but rather to general links at the Mozilla Addons.
Most extensions for Firefox, SeaMonkey, Thunderbird, and derivatives are available at the Mozilla Addons site, which hosts well over a thousand extensions. The great number of addons may make it difficult for people to find what they want.
There are many themes available, which make Firefox more attractive. The most popular themes are listed here.
Mozilla once launched Fashion Your Firefox, an easy-to-use service for users who didn't want to go through the entire library of addons to choose just a few good ones based on what they use Firefox for. Fashion Your Firefox has since long been discontinued.
The below sections herein provide links to specific extension categories at Mozilla Addons — this means that the information will always be up to date.
- Adblock Plus — Blocks or hides advertisements and other annoyances from web pages, while still retaining the correct page format. It is designed around blocklists, and allows 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 custom filters, or regular expressions for advanced ones. Adblock Plus automatically downloads a pre-set blocklist, and provides additional lists of blocklists to enable and download, based on region or function (such as cryptominers).
- Flashblock — A legacy Firefox add-on to easily block Flash, Silverlight, and HTML5 animations. The Flash file in each embed does get downloaded, but its animations are replaced with a click-to-play icon.
|Date||Compatibility||License||Alternate / developer site||Description and Notes|
|EditCSS||Pascal Guimier||Legacy||0.3.8||2011-03-15||Firefox 1.0+
|MPL 1.1||Stylesheet modifier in the sidebar. Allows editing any site's CSS on the fly.|
- Web Developer Extension — A must-have extension for anyone dabbling in web design.
|Date||Compatibility||License||Developer / company||Description and Notes|
|Download Statusbar||C.J.||Legacy||0.9.10.1-signed.1-signed||2011-10-28||Firefox 3.6–56.*||Proprietary||Enzymatic Software, LLC||View downloads in an auto-hide statusbar.|
- All the below links are very outdated, with the last update made on 14 July 2013, which makes the newest links nearly seven years old as of January 2020. They are displayed here for historical purposes.
- DownThemAll! — Easily manage downloads while accelerating them as well.
- Video DownloadHelper — Download video and audio files — Flash Video (.flv) files from sites like YouTube, MP3 files, and more.
- Cookiepie — An extension to handle multiple web accounts simultaneously, opening many Gmail, Yahoo, Hotmail accounts in different tabs on the same browser.
- Gmail Notifier — A Gmail notifier that integrates into the browser's user interface.
- WebmailCompose — Makes mailto: (email) links load a webmail mail compose window and adds a WebMailCompose link to the context menu. Now supports selected addresses.
- 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.
- Vista-aero — A theme for Firefox meant to look like Internet Explorer 7 in Windows Vista, 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.
- FoxyTunes — Adds media player controls within Firefox. Currently supports many different music players across multiple platforms.
- 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.
- Wikipedia — Provides text formatting options in the context menu for use on wiki sites based on Mediawiki, such as Wikipedia and Wikibooks.
- 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.
- 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.
- "Fashion Your Firefox". Mozilla. 2010-12-05. Archived from the original on 2010-16-13. https://web.archive.org/web/20100613132003/https://addons.mozilla.org/en-US/firefox/fashionyourfirefox/.
- Guimier, Pascal (2011-03-15). "EditCSS :: Versions". Mozilla Addons. Mozilla. Archived from the original on 2013-09-07. https://web.archive.org/web/20130907014239/https://addons.mozilla.org/en-US/firefox/addon/editcss/versions/.
- CorreyJacksinHughes / Enzymatic Software, LLC (2011-10-28). "Download Statusbar Version History". Mozilla. Archived from the original on 2018-11-15. https://web.archive.org/web/20181115124503/https://addons.mozilla.org/en-US/firefox/addon/download-statusbar/versions/.
- Enzymatic Software, LLC (2011-10-28). "Source Code License for Download Statusbar". Mozilla. Archived from the original on 2018-11-15. https://web.archive.org/web/20181115124508/https://addons.mozilla.org/en-US/firefox/addon/download-statusbar/license/0.9.10.1-signed.1-signed. "Copyright 2011 Enzymatic Software, LLC. All Rights Reserved."