Mozilla Firefox is a free web browser descended from the Mozilla Application Suite and managed by Mozilla Corporation. Firefox had 22.48% of the recorded usage share of web browsers as of April 2009, making it the second most popular browser in terms of current use worldwide, after Internet Explorer.

To display web pages, Firefox uses the Gecko layout engine, which implements some current web standards in addition to a few features which are intended to anticipate likely additions to the standards.

Firefox features include 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 (Google by default in most localizations). Functions can be added through add-ons, created by third-party developers, of which there is a wide selection, a fact that has attracted many of Firefox's users.

Firefox runs on various versions of Linux, Mac OS X, Microsoft Windows, and many other Unix-like operating systems. Firefox's source code is free software, released under a tri-license GNU GPL/GNU LGPL/MPL. Official versions are distributed under the terms of a proprietary EULA.