Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements current and anticipated web standards. The software is available for Windows, macOS, and Linux systems, with unofficial ports for other platforms, as well as a mobile version for Android (see Firefox for Android) and iOS.
Firefox is the spiritual successor of Netscape Navigator, as the Mozilla community was created by Netscape in 1998, before Netscape's acquisition by AOL. Firefox was created in 2002 under the codename "Phoenix" by members of the Mozilla community who desired a standalone browser rather than the Mozilla Application Suite bundle. During its beta phase, it proved to be popular with its testers and was praised for its speed, security, and add-ons compared to Microsoft's then-dominant Internet Explorer 6. It was released on November 9, 2004, and challenged Internet Explorer's dominance with 60 million downloads within nine months. In November 2017, Firefox began incorporating new technology under the code name "Quantum" to promote parallelism and a more intuitive user interface.
The current version of Firefox is available for Windows 10 or later versions of Windows, as well as macOS, and Linux. Its unofficial ports are available for various Unix and Unix-like operating systems, including FreeBSD, OpenBSD, NetBSD, and other operating systems, such as ReactOS. It is the default, pre-installed browser on Debian, Ubuntu, and some other Linux distros. Firefox is also available for Android and iOS. However, as with all other iOS web browsers, the iOS version uses the WebKit layout engine instead of Gecko due to platform requirements. An optimized version was also available on the Amazon Fire TV as one of the two main browsers available with Amazon's Silk Browser until April 30, 2021, when Firefox would be discontinued on that platform.
Firefox usage share grew to a peak of 32.21% in November 2009, with Firefox 3.5 overtaking Internet Explorer 7, although not all versions of Internet Explorer as a whole; its usage then declined in competition with Google Chrome. As of April 2026, according to StatCounter, it had a 4.21% usage share on traditional PCs (i.e. as a desktop browser), making it the fourth-most popular PC web browser after Google Chrome (71.56%), Microsoft Edge (11.51%), and Safari (6.17%).
Features
Features of the desktop edition include tabbed browsing, full-screen mode, spell checking, incremental search, smart bookmarks, bookmarking and downloading through drag and drop, a download manager, user profile management, private browsing, bookmark tags, bookmark exporting, offline mode, a screenshot tool, web development tools, a "page info" feature which shows a list of page metadata and multimedia items, a configuration menu at about:config for power users, and location-aware browsing (also known as "geolocation") based on a Google service. Firefox has an integrated search system which uses Google by default in most markets, but multiple search engines can be selected from a drop-down when searching (or custom ones added) and the default is easily changed. DNS over HTTPS is another feature whose default behaviour is determined geographically.
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Firefox provides an environment for web developers in which they can use built-in tools, such as the Error Console or the DOM Inspector, and extensions, such as Firebug and more recently there has been an integration feature with Pocket. Firefox Hello was an implementation of WebRTC, added in October 2014, which allows users of Firefox and other compatible systems to have a video call, with the extra feature of screen and file sharing by sending a link to each other. Firefox Hello was scheduled to be removed in September 2016.
Former features include a File Transfer Protocol (FTP) client for browsing file servers, the ability to block images from individual domains (until version 72), a 3D page inspector (versions 11 to 46), tab grouping (until version 44), and the ability to add customized extra toolbars (until version 28).
Browser extensions
Functions can be added through add-ons created by third-party developers. Add-ons are primarily coded using an HTML, CSS, JavaScript, with API known as WebExtensions, which is designed to be compatible with Google Chrome and Microsoft Edge extension systems. Firefox previously supported add-ons using the XUL and XPCOM APIs, which allowed them to directly access and manipulate much of the browser's internal functionality. As compatibility was not included in the multi-process architecture, XUL add-ons have been deemed Legacy add-ons and are no longer supported on Firefox 57 "Quantum" and newer.
Mozilla has occasionally installed extensions for users without their permission. This happened in 2017 when an extension designed to promote the show Mr. Robot was silently added in an update to Firefox.
Themes
Firefox can have themes added to it, which users can create or download from third parties to change the appearance of the browser. Firefox also provides dark, light, and system themes.
Standards
Firefox implements many web standards, including HTML, XML, XHTML, MathML, SVG 1.1 (full), SVG 2 (partial), CSS (with extensions), ECMAScript (JavaScript), DOM, XSLT, XPath, and APNG (Animated PNG) images with alpha transparency. Firefox also implements standards proposals created by the WHATWG such as client-side storage, and the canvas element. These standards are implemented through the Gecko layout engine, and SpiderMonkey JavaScript engine. Firefox 4 was the first release to introduce significant HTML5 and CSS3 support.
Firefox has passed the Acid2 standards-compliance test since version 3.0. Mozilla had originally stated that they did not intend for Firefox to pass the Acid3 test fully because they believed that the SVG fonts part of the test had become outdated and irrelevant, due to WOFF being agreed upon as a standard by all major browser makers. Because the SVG font tests were removed from the Acid3 test in September 2011, Firefox 4 and greater scored 100/100.
Safe browsing
Firefox also implements "Safe Browsing", a proprietary protocol from Google used to exchange data related to phishing and malware protection.
DRM content
Firefox has supported playback of video content protected by HTML5 Encrypted Media Extensions (EME), since version 38. For security and privacy reasons, EME is implemented within a wrapper of open-source code that allows execution of a proprietary DRM module by Adobe Systems—Adobe Primetime Content Decryption Module (CDM). CDM runs within a "sandbox" environment to limit its access to the system and provide it a randomized device ID to prevent services from uniquely identifying the device for tracking purposes. The DRM module, once it has been downloaded, is enabled and disabled in the same manner as other plug-ins. Firefox has supported Google's Widevine CDM on Windows and Mac OS X, since version 47. Mozilla justified its partnership with Adobe and Google by stating:
Firefox downloads and enables the Adobe Primetime and Google Widevine CDMs by default to give users a smooth experience on sites that require DRM. Each CDM runs in a separate container called a sandbox and you will be notified when a CDM is in use. You can also disable each CDM and opt out of future updates by following the steps below. Once you disable a CDM, however, sites using this type of DRM may not operate properly.
and that it is "an important step on Mozilla's roadmap to remove NPAPI plugin support." Upon the introduction of EME support, builds of Firefox on Windows were also introduced that exclude support for EME. The Free Software Foundation and Cory Doctorow condemned Mozilla's decision to support EME.
History
The project began as an experimental branch of the Mozilla project by Dave Hyatt, Joe Hewitt, and Blake Ross. They believed the commercial requirements of Netscape's sponsorship and developer-driven feature creep compromised the utility of the Mozilla browser. To combat what they saw as the Mozilla Suite's software bloat, they created a standalone browser, with which they intended to replace the Mozilla Suite. Version 0.1, named Phoenix, was released on September 23, 2002. On April 3, 2003, the Mozilla Organization announced that it planned to change its focus from the Mozilla Suite to Firefox and Thunderbird.
Name changes
The Firefox project has undergone several name changes. The nascent browser was originally named Phoenix, after the mythical bird that rose triumphantly from the ashes of its dead predecessor (in this case, from the "ashes" of Netscape Navigator, after it was sidelined by Microsoft Internet Explorer in the "First Browser War"). Phoenix was renamed in 2003 due to a trademark claim from Phoenix Technologies. The replacement name, Firebird, provoked an intense response from the Firebird database software project. The Mozilla Foundation reassured them that the browser would always bear the name Mozilla Firebird to avoid confusion. After further pressure, Mozilla Firebird became Mozilla Firefox on February 9, 2004. The name Firefox was said to be derived from a nickname of the red panda, which became the mascot for the newly named project. For the abbreviation of Firefox, Mozilla prefers Fx or fx, although it is often abbreviated as FF or Ff.
Versions
The Firefox project went through many versions before version 1.0 and had already gained a great deal of acclaim from numerous media outlets, such as Forbes and The Wall Street Journal. Among Firefox's popular features were the integrated pop-up blocker, tabbed browsing, and an extension mechanism for adding functionality. Although these features have already been available for some time in other browsers such as the Mozilla Suite and Opera, Firefox was the first of these browsers to have achieved large-scale adoption so quickly. Firefox attracted attention as an alternative to Internet Explorer, which had come under fire for its alleged poor program design and insecurity—detractors cite IE's lack of support for certain Web standards, use of the potentially dangerous ActiveX component, and vulnerability to spyware and malware installation. Microsoft responded by releasing Windows XP Service Pack 2, which added several important security features to Internet Explorer 6.
Version 1.0 of Firefox was released on November 9, 2004. This was followed by version 1.5 in November 2005, version 2.0 in October 2006, version 3.0 in June 2008, version 3.5 in June 2009, version 3.6 in January 2010, and version 4.0 in March 2011. From version 5 onwards, the development and release model changed into a "rapid" one; by the end of 2011 the stable release was version 9, and by the end of 2012 it reached version 17.
Project Quantum
In 2016, Mozilla announced a project known as Quantum, which sought to improve Firefox's Gecko engine and other components to improve the browser's performance, modernize its architecture, and transition the browser to a multi-process model. These improvements came in the wake of decreasing market share to Google Chrome, as well as concerns that its performance was lagging in comparison. Despite its improvements, these changes required existing add-ons for Firefox to be made incompatible with newer versions, in favor of a new extension system that is designed to be similar to Chrome and other recent browsers. Firefox 57, which was released in November 2017, was the first version to contain enhancements from Quantum, and has thus been named Firefox Quantum. A Mozilla executive stated that Quantum was the "biggest update" to the browser since version 1.0. Unresponsive and crashing pages only affect other pages loaded within the same process. While Chrome uses separate processes for each loaded tab, Firefox distributes tabs over four processes by default (since Quantum), to balance memory consumption and performance. The process count can be adjusted, where more processes increase performance at the cost of memory, therefore suitable for computers with larger RAM capacity.
Guest session
In 2013, Firefox for Android added a guest session mode, which wiped browsing data such as tabs, cookies, and history at the end of each guest session. Guest session data was kept even when restarting the browser or device, and deleted only upon a manual exit. The feature was removed in 2019, purportedly to "streamline the experience".
Other
On May 3, 2019, the expiry of an intermediate signing certificate on Mozilla servers caused Firefox to automatically disable and lock all browser extensions (add-ons). Mozilla began the roll-out of a fix shortly thereafter, using their Mozilla Studies component.
On January 6, 2021, Firefox ended support for Adobe Flash, following the release of Firefox 85.
On June 1, 2021, Firefox's 'Proton' redesign was offered through its stable release channel after being made available in the beta builds. While users were initially allowed to revert to the old design through about:config, the corresponding key-value pairs reportedly stopped working in later builds, resulting in criticism. These included accessibility concerns despite Mozilla's claim to "continue to work with the accessibility community" and had not been resolved as of October 2024.
On January 13, 2022, an issue with Firefox's HTTP/3 implementation resulted in a widespread outage for several hours.
On September 26, 2023, Firefox 118.0 introduced on-device translation of web page content. The translation feature builds on the work of European Union-funded Project Bergamot, that Mozilla participated in.
On January 23, 2024, along with the release of Firefox 122.0, Mozilla introduced an official APT repository for Debian-based Linux distributions.
Security
From its inception, Firefox was positioned as a security-focused browser. At the time, Internet Explorer, the dominant browser, was facing a security crisis. Multiple vulnerabilities had been found, and malware like Download.Ject could be installed simply by visiting a compromised website. The situation was so severe that the U.S. government reportedly issued warnings about security risks with Internet Explorer. Firefox, being less integrated with the operating system, was considered a safer alternative since it was less likely to have issues that could completely compromise a computer. This led to a significant increase in Firefox's popularity during the early 2000s as a more secure alternative. Moreover, Firefox was considered to have fewer actively exploitable security vulnerabilities compared to its competitors. In 2006, The Washington Post reported that exploit code for known security vulnerabilities in Internet Explorer was available for 284 days compared to only nine days for Firefox before the problem was fixed. A Symantec study around the same period showed that even though Firefox had a higher number of vulnerabilities, on average vulnerabilities were fixed faster in Firefox than in other browsers during that period.
During this period, Firefox used a monolithic architecture, like most browsers at the time. This meant all browser components ran in a single process with access to all system resources. This setup had multiple security issues. If a web page used too many resources, the entire Firefox process would hang or crash, affecting all tabs. Additionally, any exploit could easily access system resources, including user files. Between 2008 and 2012, most browsers shifted to a multiprocess architecture, isolating high-risk processes like rendering, media, GPU, and networking. However, Firefox was slower to adopt this change. It wasn't until 2015 that Firefox started its Electrolysis (e10s) project to implement sandboxing across multiple components. This rewrite relied on interprocess communication using Chromium's interprocess communication library and placed various components, including the rendering component, in its sandbox. Firefox released this rewrite in to beta in August 2016, noting a 10–20% increase in memory usage, which was lower than Chrome's at the time. However, the rewrite caused issues with their legacy extension API, which was not designed to work cross-process and required shim code to function correctly. After over a year in beta, the rewrite was enabled by default all users of Firefox in November 2017.
In 2012, Mozilla launched a new project called Servo to write a completely new and experimental browser engine utilizing memory safe techniques written in Rust. In 2018, Mozilla opted to integrate parts of the Servo project into the Gecko engine in a project codenamed the Quantum project. The project completely overhauled Firefox's page rendering code, resulting in performance and stability gains while also improving the security of existing components. Additionally, the older incompatible extension API was removed in favour of a WebExtension API that more closely resembled Google Chrome's extension system. This broke compatibility with older extensions but resulted in fewer vulnerabilities and a much more maintainable extension system. While the Servo project was intended to replace more parts of the Gecko Engine, this plan never came to fruition. In 2020, Mozilla laid off all developers on the Servo team transferring ownership of the project to the Linux Foundation.
In 2026, in collaboration with Anthropic, Mozilla used Claude Opus 4.6 to detect 22 vulnerabilities in Firefox over 2 weeks. 14 of them were high-severity, making almost a fifth of the number of high-severity Firefox vulnerabilities fixed during the full year 2025. In addition, the tool also detected other 90 non-security bugs. All the vulnerabilities and most of the bugs were quickly fixed.
Privacy
When Firefox was initially released, it used a custom script permission policy where scripts that were signed by the page could gain access to higher privilege actions, such as the ability to set a user's preferences. However, this model was not widely used and was later discontinued by Firefox. Modern day Firefox instead follows the standard same-origin policy permission model that is followed by most modern browsers which disallows scripts from accessing any privileged data including data about other websites.
It uses TLS to protect communications with web servers using strong cryptography when using the HTTPS protocol. The freely available HTTPS Everywhere add-on enforces HTTPS, even if a regular HTTP URL is entered; this feature is now part of the browser. Firefox supports HTTP/2.
In February 2013, plans were announced for Firefox 22 to disable third-party cookies by default. However, the introduction of the feature was then delayed so Mozilla developers could "collect and analyze data on the effect of blocking some third-party cookies." Mozilla also collaborated with Stanford University's "Cookie Clearinghouse" project to develop a blacklist and whitelist of sites that will be used in the filter.
Version 23, released in August 2013, followed the lead of its competitors by blocking iframe, stylesheet, and script resources served from non-HTTPS servers embedded on HTTPS pages by default. Additionally, JavaScript could also no longer be disabled through Firefox's preferences, and JavaScript was automatically re-enabled for users who upgraded to 23 or higher with it disabled. The change was made because JavaScript was being used across a majority of websites on the web, and disabling JavaScript could potentially have untoward repercussions on inexperienced users who are unaware of its impact. Firefox also cited the fact that extensions like NoScript, which can disable JavaScript in a more controlled fashion, were widely available. The following release added the ability to disable JavaScript through the developer tools for testing purposes.
Beginning with Firefox 48, all extensions must be signed by Mozilla to be used in release and beta versions of Firefox. Firefox 43 blocked unsigned extensions but allowed enforcement of extension signing to be disabled. All extensions must be submitted to Mozilla Add-ons and be subject to code analysis to be signed, although extensions do not have to be listed on the service to be signed. On May 2, 2019, Mozilla announced that it would be strengthening the signature enforcement with methods that included the retroactive disabling of old extensions now deemed to be insecure.
Since version 60 Firefox includes the option to use DNS over HTTPS (DoH), which causes DNS lookup requests to be sent encrypted over the HTTPS protocol. To use this feature the user must set certain preferences beginning with "network.trr" (Trusted Recursive Resolver) in about:config: if network.trr.mode is 0, DoH is disabled; 1 activates DoH in addition to unencrypted DNS; 2 causes DoH to be used before unencrypted DNS; to use only DoH, the value must be 3. By setting network.trr.uri to the URL, special Cloudflare servers will be activated. Mozilla has a privacy agreement with this server host that restricts its collection of information about incoming DNS requests.
On May 21, 2019, Firefox was updated to include the ability to block scripts that used a computer's CPU to mine cryptocurrency without a user's permission, in Firefox version 67.0. The update also allowed users to block known fingerprinting scripts that track their activity across the web, however it does not resist fingerprinting on its own. In February 2021, Firefox launched Total Cookie Protection in version 86 to offer protection against cross-site tracking, without breaking the websites users visit. Also known as state partitioning, Total Cookie Protection works by creating a separate "cookie jar" for each website, thereby preventing data from being shared between websites. The feature, which was named one of 2021's best security innovations by Popular Science, also isolates local storage, service workers, and other common ways for sites to store data. Total Cookie Protection was enabled by default in 2022.
Localizations
Firefox is a widely localized web browser. Mozilla uses the in-house Pontoon localization platform. The first official release in November 2004 was available in 24 different languages and for 28 locales. In 2019, Mozilla released Project Fluent a localization system that allows translators to be more flexible with their translation than to be constrained in one-to-one translation of strings. As of June 2026, the supported versions of Firefox are available in 97 locales (88 languages).
Platform availability
There are desktop versions of Firefox for Microsoft Windows, macOS, and Linux, while Firefox for Android is available for Android (formerly Firefox for mobile, it also ran on Maemo, MeeGo, and Firefox OS) and Firefox for iOS is available for iOS. Smartphones that support Linux but not Android, or iOS apps can also run Firefox in its desktop version, for example using postmarketOS, Mobian or Ubuntu Touch.
Notes
In March 2014, the Windows Store app version of Firefox was cancelled, although there is a beta release.
SSE2 instruction set support is required for 49.0 or later for Windows and 53.0 or later for Linux, IA-32 support only applies to superscalar processors.
