Mozilla Firefox (abbreviated officially as Fx, but also commonly as FF) is a free web browser descended from the Mozilla Application Suite, managed by the Mozilla Corporation. Firefox had 19.03% of the recorded usage share of web browsers as of June 2008, making it the second-most popular browser in current use worldwide, after Internet Explorer.

Firefox uses the free Gecko layout engine, which implements some current web standards plus a few features which are intended to anticipate likely additions to the standards.

Firefox includes tabbed browsing, a spell checker, incremental find, live bookmarking, a download manager, and an integrated search system that uses the user's desired search engine. Functions can be added through around 2,000 add-ons created by third-party developers, the most popular of which include NoScript (script blocker), Tab Mix Plus (adds many customizable options to tabs), FoxyTunes (controls music players), Adblock Plus (ad blocker), StumbleUpon (website discovery), DownThemAll! (download functions) and Web Developer (web tools).

Firefox runs on various versions of Microsoft Windows, Mac OS X, Linux, and many other Unix-like operating systems. Its current stable release is version 3.0, released on June 17, 2008. Firefox's source code is free software, released under a tri-license GPL/LGPL/MPL. A portable version of Firefox is available from PortableApps.com.

Features included with Firefox are tabbed browsing, spell checker, incremental find, live bookmarking, an integrated download manager, and an integrated search system that uses the user's desired search engine. The developers of Firefox aimed to produce a browser that "just surfs the web" and delivers the "best possible browsing experience to the widest possible set of people."

Users can customize Firefox with extensions and themes. Mozilla maintains an add-on repository at addons.mozilla.org with nearly 2000 add-ons in it as of September 2007.

Firefox provides an environment for web developers in which they can use built-in tools, such as the Error Console or the DOM Inspector, or extensions, such as Firebug.

Standards support

Mozilla Firefox supports many web standards, including HTML, XML, XHTML, SVG 1.1 (partial), CSS (with extensions), ECMAScript (JavaScript), DOM, MathML, DTD, XSLT, XPath, and PNG images with alpha transparency. Firefox also supports standards proposals created by the WHATWG such as client-side storage, and canvas element.

Firefox passes the Acid2 standards-compliance test from version 3.0. Firefox 3.0 does not pass the Acid3 test; it scores 71/100 and does not render the image correctly.

Security

Firefox uses a sandbox security model, and limits scripts from accessing data from other web sites based on the same origin policy. It uses SSL/TLS to protect communications with web servers using strong cryptography when using the https protocol. It also provides support for web applications to use smartcards for authentication purposes.

The Mozilla Foundation offers a "bug bounty" to researchers who discover severe security holes in Firefox. Official guidelines for handling security vulnerabilities discourage early disclosure of vulnerabilities so as not to give potential attackers an advantage in creating exploits.

Because Firefox has fewer and less severe publicly known unpatched security vulnerabilities than Internet Explorer, improved security is often cited as a reason to switch from Internet Explorer to Firefox. The Washington Post reports that exploit code for critical unpatched security vulnerabilities in Internet Explorer was available for 284 days in 2006. In comparison, exploit code for critical security vulnerabilities in Firefox was available for 9 days before Mozilla shipped a patch to remedy the problem.

A 2006 Symantec study showed that although Firefox had surpassed other browsers in the number of vendor-confirmed vulnerabilities that year through September, these vulnerabilities were patched far more quickly than those found in other browsers. Symantec later clarified their statement, saying that Firefox still had fewer security vulnerabilities than Internet Explorer, as counted by security researchers. As of June 7, 2008, Firefox 2 has three security vulnerabilities unpatched, the most severe of which was rated "less critical" by Secunia. Internet Explorer 7 has ten security vulnerabilities unpatched, the most severe of which was rated "Moderately critical" by Secunia.