Mozilla Firefox (or Firefox for short) is a free open source web browser developed by Mozilla foundation and its subsidiary Mozilla Corporation. Firefox can be used for windows, Mac OS and Linux operating systems, while its Firefox for Android can be used for Android (formerly known as Firefox for mobile, which has also been running on the discontinued Firefox OS); all of these versions use gecko layout engine to render web pages, which implements current and expected network standards. Another version of Firefox for IOS was released in late 2015, but due to Apple's restrictions, third-party Web browsers can only use IOS's built-in WebKit based layout engine, so this version does not use gecko.

Firefox was created by members of the Mozilla community in 2002 with the name "phoenix" and they want to use a separate browser instead of the Mozilla application suite bundle. Even in the testing phase, Firefox is still popular with testers, and compared with Microsoft's then dominant Internet Explorer 6, its speed, security and add-on components are also praised. Firefox was released in November 2004 and downloaded 1 million times in nine months with 60, the first time Internet Explorer's dominance has been challenged. Firefox is seen as the spiritual successor to Netscape Navigator because the Mozilla community was created by Netscape in 1998 and then acquired by AOL.

By the end of 2009, Firefox's usage had grown to a peak of 32%, making version 3.5 the world's most popular browser for the time being. Usage then fell in competition with Google Chrome. As of January 2016, Firefox accounted for 9-16% of the world's "desktop" browser usage, making it the second most popular web browser. Firefox is still the most popular desktop browser in Cuba, Eritrea and Germany (even the most popular desktop browser), accounting for 85.93%, 79.39% and 37.01% 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 500 million Firefox users worldwide.