Mozilla Firefox is a free, open source, cross-platform, graphical web browser developed by the Mozilla Corporation and hundreds of volunteers.

Before its 1.0 release, Firefox had already gained acclaim from numerous media outlets, including Forbes and the Wall Street Journal. With over 25 million downloads in the 99 days after the initial 1.0 release, Firefox became one of the most downloaded free and open source applications, especially among home users. On October 19, 2005, Firefox had its 100 millionth download, just 344 days after the release of version 1.0. Firefox 1.5 was released on November 29, 2005, with more than 2 million downloads within the first 36 hours.

Firefox includes an integrated pop-up blocker, tabbed browsing, live bookmarks, support for open standards, and an extension mechanism for adding functionality. Although other browsers have introduced these features, Firefox became the first such browser to achieve wide adoption.

Firefox has attracted attention as an alternative to other browsers such as Microsoft Internet Explorer. As of January 2006, estimates suggest that Firefox's usage share is around 10% of overall browser usage.