Mozilla Firefox (known simply as Firefox) is a free and open-source^[17] web browser developed for Windows, OS X, and Linux, with a mobile version for Android, by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. Firefox uses the Gecko layout engine to render web pages, which implements current and anticipated web standards.^[18]

As of February 2014, Firefox has between 12% and 22% of worldwide usage, making it, per different sources, the third most popular web browser.^{[19][20][21][22]} According to Mozilla, as of December 2014 Firefox counts half a billion users around the world.^[23] The browser has had particular success in Indonesia, Iran, Germany, and Poland, where it is the most popular browser with 55%,^[24] 46%,^[25] 43%,^[26] and 41%^[27] of the market share, respectively.