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Mozilla Firefox

Summary

Mozilla Firefox is a free, open-source web browser made by Mozilla and descended from the Netscape source code released in 1998. It began as Phoenix in September 2002, was renamed Firebird in 2003 and Firefox on February 9, 2004, and reached version 1.0 on November 9, 2004. It draws pages with Mozilla's Gecko engine, and as of September 2026 StatCounter put it at 2.8% of page views worldwide.

Web Building Encyclopedia · Software and platforms · Last reviewed October 22, 2026 · 10 min read

What Firefox is

Mozilla Firefox is a free, open-source web browser. Mozilla’s terms of use call it “free and open source web browser software” and say its source code is published under several open-source licenses, the Mozilla Public License among them.1

Firefox descends directly from Netscape Navigator. Netscape released its browser’s source code in 1998, the Mozilla project grew up around that code, and in 2002 community members released a smaller browser built from it, first called Phoenix and later renamed Firefox.23

Firefox draws pages with Gecko, Mozilla’s own layout engine. MDN, Mozilla’s developer reference, explains that a layout engine interprets HTML, CSS, JavaScript and embedded content such as images and draws them on screen. Gecko also contains a networking stack, a graphics stack and a JavaScript virtual machine, the part that runs scripts.4

There is one exception. On iPhone and iPad, Apple’s App Store rules say apps that browse the web must use Apple’s WebKit framework, unless the developer receives an entitlement to use another engine, which Apple offers for the EU and Japan.5 Mozilla’s source documentation says Firefox for iOS is built natively for Apple’s system, does not use Gecko, and shows web pages through WebKit.6

As of September 2026, StatCounter put Firefox at 2.8% of page views worldwide across all devices, fourth after Chrome, Safari and Edge.7

From Netscape’s code to the Mozilla Suite (1998 to 2003)

On January 22, 1998, Netscape made Navigator and Communicator Standard Edition 4.0 free for all users and said it would post the source code of its next browser, Communicator 5.0. Source code is the human-readable text of a program. Netscape expected the first developer release by the end of March 1998.8

Chief executive Jim Barksdale said that by giving the code away, “we can ignite the creative energies of the entire Net community.”8 A month later, on February 23, 1998, Netscape announced mozilla.org, a dedicated team and website to run the open-source project.2

The first developer release came on March 31, 1998, through mozilla.org. It contained code for the next versions of Navigator and the Composer page editor, for Linux, 32-bit Windows and the Power PC Macintosh, under a new license called the Netscape Public License.9 In September 2001 the project relicensed more than 6,000 of those files under a choice of three licenses.2

A new layout engine

The old Netscape code proved hard to build on. On October 26, 1998, Brendan Eich published a roadmap that stopped coordinated work on the old layout code and moved the project to a new engine, NGLayout. He wrote that the features content developers most wanted, including correct CSS and a Level 1 DOM (the standard way for scripts to reach every element of a page), had been hard to add to the old code.10

A mozilla.org note dated December 10, 1998 said the press was writing about “Gecko,” a Netscape marketing name covering two Mozilla projects: NGLayout, earlier code-named Raptor, and XPFE, the cross-platform front end. The note said mozilla.org, being separate from Netscape, would keep calling them NGLayout and XPFE.11 Gecko became the name that stayed.12

Netscape 6, built on the Mozilla browser suite, came out at the Fall Comdex trade show on November 14, 2000. Linux Journal described its engine as designed to follow W3C standards strictly, with a quirks mode for older pages.13

Mozilla 1.0

The project reached version 1.0 on June 5, 2002. Mozilla 1.0 was a suite combining a web browser, an email reader and a chat client, built on Gecko. The announcement credited thousands of programmers and tens of thousands of testers.12 Mozilla’s own history says few people used it, and that by 2002 well over 90% of internet users browsed with Internet Explorer.3

Phoenix, Firebird, Firefox (2002 to 2004)

Phoenix 0.1 was released on September 23, 2002, and Mozilla counts it as the first version of Firefox.14 Its release notes describe it as a redesign of the Mozilla browser component, written in XUL, Mozilla’s language for describing program interfaces, and built to run on several operating systems.15

The release notes list a toolbar that could be rearranged, a quick search for bookmarks and history, and a rebuilt bookmarks manager with undo and redo. They say speed was one of its main design goals.15

Two renamings

The name Phoenix lasted about seven months. A mozilla.org status report of April 18, 2003 said it had been announced that week that Phoenix’s new name was Firebird. The change answered a request from Phoenix Technologies, a maker of BIOS software (the startup code inside a computer) that also made an embedded browser.16

Firebird did not last either. On February 9, 2004, Mozilla Firebird was renamed Mozilla Firefox.2 Mozilla’s naming FAQ said the change was made to avoid overlap, because another open-source project already used the name Firebird, and it noted that a firefox is another name for the red panda.17

The browser becomes the main product

In a roadmap dated April 2, 2003, Brendan Eich and David Hyatt moved the project’s focus from the suite to standalone programs: a browser and a separate mail program, Thunderbird. The archived copy, last revised in 2005, names them Firefox and Thunderbird and calls them the project’s leading products.18

The roadmap gave its reasons. The standalone browser was smaller and faster, largely because features could be added through extensions instead of being built in, and the integrated suite had been costly to maintain. The suite itself was kept on maintenance, with security updates, for businesses that still relied on it.18

On July 15, 2003, mozilla.org became the Mozilla Foundation, an independent nonprofit. AOL pledged $2 million over two years, and Mitch Kapor, the foundation’s chairman, gave $300,000. Mitchell Baker became its president.19 The same day, Mozilla’s archive records, AOL cut its remaining Mozilla developers.2

Firefox 1.0, tabs and extensions (2004 to 2012)

The Mozilla Foundation released Firefox 1.0 on November 9, 2004, for Windows, Mac OS X and Linux, in more than a dozen languages. It said the preview release had been downloaded more than eight million times in the previous month.20 Baker said: “Now millions more will be able to enjoy a better web experience.”20

The release listed these features, among others:20

  • Tabbed browsing. Several pages open in one window, each under its own tab.
  • Pop-up blocking. The user decides when a site may open pop-up windows.
  • Built-in search. A search box for services such as Google, Yahoo, eBay, Amazon and Dictionary.com.
  • Live Bookmarks. Bookmarks that show the latest headlines from news sites and blogs.
  • Phishing and spoofing protection. Clear display of the true identity of secure sites.
  • Extensions. More than 100 add-ons, including toolbars from A9.com, Google and Yahoo. An extension is a small program that adds a feature to the browser.

The New York Times ad

Firefox’s supporters also helped promote the launch. On December 15, 2004, the foundation announced a two-page ad in the next day’s edition of The New York Times, listing the names of the thousands of people worldwide who had given to its fundraising campaign for the 1.0 launch.21

The ad was coordinated by Spread Firefox, the volunteer advocacy site, which had more than 50,000 registered members, and its design was led by volunteer Christopher Messina. By then, the foundation said, more than 11 million people had downloaded Firefox.21

Growth

Mozilla says Firefox 1.0 was downloaded more than 100 million times in less than a year.3 On August 3, 2005, the foundation formed the Mozilla Corporation, a taxable subsidiary it wholly owns, to make Firefox and Thunderbird. It said Firefox had passed 75 million downloads and was close to 10% of global internet usage.22

The older suite stepped aside. On March 10, 2005, the foundation said it would stop official releases of the Mozilla Application Suite.2 Volunteers carry it on as SeaMonkey, which describes itself as a community effort and released version 2.53.24 on July 28, 2026.23

Netscape’s own browser ended on Mozilla’s. When AOL announced on December 28, 2007 that it would stop developing Netscape browsers, it recommended that users download Firefox. Net Applications counted Firefox at 16% of browser use the month before, against 77.4% for Internet Explorer.24

In May 2012, the month Google Chrome first passed Internet Explorer for a full month, StatCounter put Firefox at 25.55% of browser use worldwide.25 That contest is covered in the entry on the browser wars.

Gecko and Firefox Quantum (2017)

Under a program called Project Quantum, Mozilla moved code from Servo, its experimental browser engine, into Firefox. In August 2017 Lin Clark of Mozilla described Quantum CSS, earlier called Stylo, as the first major part of Servo to make that move.26

Servo is written in the Rust programming language, and Rust’s compiler helps prevent data races, bugs in which two parts of a program change the same data at once. Quantum CSS used that safety to split the work of styling a page across several processor cores.26

Firefox 57, released on November 14, 2017, was the version Mozilla launched as Firefox Quantum.27 Its release notes list a completely new browsing engine and a redesigned interface, and they let users turn on tracking protection at all times.28

Mark Mayo of Mozilla wrote that Firefox Quantum was “over twice as fast as Firefox from 6 months ago” and used less memory than competing browsers. He said it gave the tab in front of the user priority over the others, and that it built on the multi-process design Firefox had launched that June.27

Firefox 57 also changed extensions. From that release Firefox supported only extensions built with the WebExtension API, a programming interface for add-ons, and older legacy extensions stopped working.28

Release timing changed two years later. In September 2019, Mozilla said major releases, then shipped every six to eight weeks, would move to a four-week cycle starting with Firefox 71. Extended Support Release (ESR) versions for businesses would stay on a yearly schedule.29

Privacy features

Firefox has added several layers of protection against tracking, the practice of following a person from site to site, often with third-party cookies (small files set by a site other than the one being visited).

Enhanced Tracking Protection

Firefox 69, released on September 3, 2019, turned Enhanced Tracking Protection on by default. Its standard setting blocked third-party tracking cookies and cryptominers, scripts that use a visitor’s computer to mine cryptocurrency. An optional strict setting also blocked fingerprinters, which identify a visitor from details of the device and browser instead of a cookie.30

As of October 2026, Mozilla’s support pages say the Standard mode, on from the first download, blocks social media trackers, cross-site tracking cookies, cryptominers and fingerprinters on all sites, and tracking content in private windows. Strict mode blocks all cross-site cookies and tracking content in every window, and Mozilla warns that it can break some sites.31

A shield icon to the left of the address bar shows whether Firefox is blocking anything on the current site. Clicking it lets the user switch the protection off for that one site, which adds the site to an exception list.31

Total Cookie Protection and SmartBlock

On June 14, 2022, Mozilla made Total Cookie Protection a default for all Firefox desktop users worldwide. It gives each website its own separate “cookie jar,” so a cookie set while visiting one site cannot be read on another.32

Blocking scripts can break pages, so Firefox 87, in March 2021, added SmartBlock. In private windows and Strict mode it replaces some blocked third-party tracking scripts with local stand-ins that behave just enough like the originals to keep pages working.33

Firefox in 2026

Firefox’s share depends heavily on the device. StatCounter’s figures for September 2026, measured in page views:

MeasureFirefox shareLeader
Worldwide, all devices2.8%7Chrome, 66.36%
Worldwide, desktop6.11%34Chrome, 65.09%
Worldwide, mobile0.69%35Chrome, 67.64%
United States, all devices3.33%36Chrome, 53.82%

On desktop computers Firefox ranks fourth, after Chrome, Edge and Safari. On phones it ranks sixth, after Chrome, Safari, Samsung Internet, Opera and UC Browser.3435

Mozilla’s release calendar lists Firefox 158 as released on October 13, 2026, with Firefox 159 due on October 27, 2026. The matching Extended Support Release is 153.5.37 A fix release, 158.0.1, followed on October 20, 2026. Among other changes, it blocked compromised intermediate certificates from the certificate authority eMudhra, even when Firefox’s remote settings service is turned off or unavailable.38

Mozilla also publishes MDN Web Docs, a reference for HTML, CSS, JavaScript and web APIs. MDN began in 2005 as DevMo, a community wiki, took the name Mozilla Developer Network in 2010, and reports more than 15 million users a month.39

Timeline

  1. Firefox 158.0.1 blocks compromised eMudhra intermediate certificates38
  2. Total Cookie Protection becomes a default for all Firefox desktop users worldwide32
  3. Firefox 69 turns Enhanced Tracking Protection on by default30
  4. Firefox 57, launched as Firefox Quantum, supports only WebExtension add-ons28
  5. AOL ends Netscape browser development and recommends Firefox24
  6. The Mozilla Foundation forms the Mozilla Corporation to make Firefox and Thunderbird22
  7. The foundation says it will stop official Mozilla Application Suite releases2
  8. A two-page ad listing donors to the Firefox 1.0 launch campaign runs in The New York Times21
  9. Firefox 1.0 is released for Windows, Mac OS X and Linux20
  10. Mozilla Firebird is renamed Mozilla Firefox2
  11. The Mozilla Foundation is launched as an independent nonprofit19
  12. Phoenix is renamed Firebird after a request from Phoenix Technologies16
  13. Phoenix 0.1, the first version of Firefox, is released14
  14. Mozilla 1.0, the browser suite built on Gecko, is released12
  15. Netscape's first developer release of its browser source code goes out through mozilla.org9

What it means for your website

Firefox had 2.8% of page views worldwide in September 2026,7 but it is the main browser built on Gecko, and its privacy defaults affect what your site can measure. A few checks follow.

  • Test in Gecko, not only in Chrome. Baseline, the browser support label on Google’s web.dev site, counts Firefox on desktop and Android among its core browsers, with Chrome, Edge and Safari.40 Open new pages, forms and menus in Firefox before launch. Developers can automate this: the testing tool Playwright runs the same tests on Chromium, WebKit and a Firefox build that matches the recent stable release.41
  • Check your desktop visitors. Firefox had 6.11% of desktop page views worldwide in September 2026, against 0.69% on phones.3435 If your analytics show many desktop visitors, Firefox matters more to you than the global figure suggests.
  • Expect gaps in tracking. Firefox blocks cross-site tracking cookies, social media trackers and fingerprinters by default.31 Some visits and conversions may not reach your analytics or ad reports. Our page on websites and Google Ads explains how conversion tracking works.
  • Test embedded tools. Total Cookie Protection keeps each site’s cookies in a separate jar.32 A booking widget, chat box, payment form or login from another company may behave differently inside your page. Try each one in Firefox, then again with Strict mode switched on, since Mozilla warns that Strict mode can break some sites.31
  • Keep your certificate current. Firefox 158.0.1 blocked compromised eMudhra intermediate certificates.38 If your certificate was issued through one of those intermediates, Firefox visitors may see warnings. See our news report on the eMudhra certificate keys and the glossary entry on the SSL certificate.
  • Use MDN when you check code. Mozilla’s MDN documents HTML, CSS, JavaScript and web APIs, so it is a useful second opinion when a designer or plugin vendor says a feature works in every browser.39

See also

References

  1. Mozilla Firefox Terms of Use (effective September 16, 2026). Mozilla. 2026-09-16. accessed October 22, 2026.
  2. History of the Mozilla Project (archived mozilla.org history). Mozilla. accessed October 22, 2026.
  3. History of the Mozilla Project. Mozilla. accessed October 22, 2026.
  4. Gecko (glossary). MDN Web Docs, Mozilla. accessed October 22, 2026.
  5. App Review Guidelines, guideline 2.5.6. Apple Developer. accessed October 22, 2026.
  6. Firefox for iOS. Firefox Source Docs, Mozilla. accessed October 22, 2026.
  7. Browser Market Share Worldwide (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  8. Netscape Announces Plans to Make Next-Generation Communicator Source Code Available Free on the Net (press release, comp.os.os2.announce archive). Netscape Communications, via MIT. 1998-01-22. accessed October 22, 2026.
  9. Netscape Accelerates Communicator Evolution with First Release of Next-Generation Communicator Source Code (press release). Netscape Communications, via Cover Pages. 1998-03-31. accessed October 22, 2026.
  10. Development Roadmap (Brendan Eich). mozilla.org (archived). 1998-10-26. accessed October 22, 2026.
  11. Gecko (mozilla.org note on the NGLayout name). mozilla.org (archived). 1998-12-10. accessed October 22, 2026.
  12. mozilla.org Launches Mozilla 1.0. Mozilla press releases. 2002-06-05. accessed October 22, 2026.
  13. Mozilla Browser Suite (Mike Angelo). Linux Journal. 2000-11-29. accessed October 22, 2026.
  14. Milestone: Phoenix 0.1 released, first version of Firefox. Mozilla Community blog. 2013-05-13. accessed October 22, 2026.
  15. Phoenix 0.1 (Pescadero) release notes. mozilla.org, via Mozilla website archive. accessed October 22, 2026.
  16. mozilla.org status update, April 18, 2003. mozilla.org (archived). 2003-04-18. accessed October 22, 2026.
  17. Mozilla Firefox: brand name FAQ. mozilla.org (archived). accessed October 22, 2026.
  18. Mozilla Development Roadmap (Brendan Eich and David Hyatt). mozilla.org (archived). 2003-04-02. accessed October 22, 2026.
  19. America Online Pledges $2 Million to Help Launch Independent Non-Profit (Mozilla Foundation launch). Mozilla. 2003-07-15. accessed October 22, 2026.
  20. Open Source Software Driving Innovations in Web Experience (Firefox 1.0 release). Mozilla Foundation. 2004-11-09. accessed October 22, 2026.
  21. Mozilla Foundation Places Two-Page Advocacy Ad in The New York Times. Mozilla press releases. 2004-12-15. accessed October 22, 2026.
  22. Mozilla Foundation Forms New Organization to Further the Creation of Free, Open Source Internet Software, Including the Award-Winning Mozilla Firefox Browser. Mozilla press releases. 2005-08-03. accessed October 22, 2026.
  23. SeaMonkey. The SeaMonkey Project. accessed October 22, 2026.
  24. R.I.P.: AOL kills Netscape (Gregg Keizer). Computerworld. 2007-12-28. accessed October 22, 2026.
  25. Independent web analytics firm StatCounter confirms milestone as Chrome overtakes IE globally for first calendar month. StatCounter Global Stats. 2012-06-01. accessed October 22, 2026.
  26. Inside a super fast CSS engine: Quantum CSS (aka Stylo) (Lin Clark). Mozilla Hacks. 2017-08-22. accessed October 22, 2026.
  27. Introducing the New Firefox: Firefox Quantum (Mark Mayo). The Mozilla Blog. 2017-11-14. accessed October 22, 2026.
  28. Firefox 57.0 release notes. Mozilla. 2017-11-14. accessed October 22, 2026.
  29. Moving Firefox to a faster 4-week release cycle. Mozilla Hacks. 2019-09. accessed October 22, 2026.
  30. Firefox 69.0 release notes. Mozilla. 2019-09-03. accessed October 22, 2026.
  31. Enhanced Tracking Protection in Firefox for desktop. Mozilla Support. accessed October 22, 2026.
  32. Firefox rolls out Total Cookie Protection by default to more users worldwide. The Mozilla Blog. 2022-06-14. accessed October 22, 2026.
  33. Introducing SmartBlock. Mozilla Security Blog. 2021-03-23. accessed October 22, 2026.
  34. Desktop Browser Market Share Worldwide (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  35. Mobile Browser Market Share Worldwide (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  36. Browser Market Share United States of America (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  37. Firefox release calendar. Mozilla release management, whattrainisitnow.com. accessed October 22, 2026.
  38. Firefox 158.0.1 release notes. Mozilla. 2026-10-20. accessed October 22, 2026.
  39. About MDN. MDN Web Docs, Mozilla. accessed October 22, 2026.
  40. Baseline. web.dev, Google. accessed October 22, 2026.
  41. Browsers. Playwright documentation. accessed October 22, 2026.

Web Building Encyclopedia. First published October 22, 2026. Last reviewed October 22, 2026. Every fact on this page is linked to a source we opened. Spotted an error? Write to [email protected].