Mozilla Firefox is a free and open source web browser descended from the Mozilla Application Suite, managed by the Mozilla Corporation. Firefox had 19.46% of the recorded usage share of web browsers as of September 2008, making it the second-most popular browser in current use worldwide, after Internet Explorer.

To display web pages, Firefox uses the 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 add-ons created by third-party developers, the most popular of which include the NoScript JavaScript disabling utility, Tab Mix Plus customizer, FoxyTunes media player control toolbar, Adblock Plus ad blocking utility, StumbleUpon (website discovery), DownThemAll! download enhancer and Web Developer toolbar.

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 3.0.3, released on September 26, 2008. Firefox's source code is free software, released under a tri-license GPL/LGPL/MPL.