Mozilla Firefox (or simply Firefox) is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. Firefox is available for Windows, OS X and Linux operating systems, with its mobile versions available for Android, and Firefox OS; where all of these versions use the Gecko layout engine to render web pages, which implements current and anticipated web standards, but an additional version released in late 2015 – Firefox for iOS has also been made available – that doesn't use Gecko.

Firefox was created in 2002, under the name "Phoenix" by the Mozilla community members who wanted a standalone browser rather than the Mozilla Application Suite bundle. Even during its beta phase, Firefox proved to be popular by its testers and was praised for its speed, security and add-ons compared to Microsoft's then-dominant Internet Explorer 6. Firefox was released in November 2004, and was highly successful with 60 million downloads within nine months, which was the first time that Internet Explorer's dominance was challenged. Firefox is considered the spiritual successor of Netscape Navigator, as the Mozilla community was created by Netscape in 1998 before their acquisition by AOL.

As of January 2016, Firefox has between 9% and 16% of worldwide usage as a "desktop" browser, making it the second most popular web browser. Firefox is the most popular browser in Cuba, Eritrea and Germany, with 85.93% and 79.39% and 38.36%, of the market share, respectively. It is also the most popular desktop browser in many other African countries. According to Mozilla, as of December 2014 there were half a billion Firefox users around the world. With Internet Explorer declining, Firefox reached second place in February 2016, as a desktop browser.