Mozilla Firefox is a free, open source web browser developed by the Mozilla Foundation. The software is licensed under the Mozilla Public License version 1.1 and is the main competitor of Microsoft's Internet Explorer.
Firefox was originally developed as an offshoot of the Mozilla Browser project, developed by Netscape, as a lightweight version of the software. The original Mozilla browser was extremely feature rich, including such things as an HTML editor, which created a large and correspondingly slow piece of software. Firefox has gone through several name changes, all due to existing trademarks. It was originally named Phoenix, then Firebird and finally Firefox.
Since it's original release in 2003, Firefox has had a continued increase in the market share of web browsers. Exact numbers are difficult to determine, but as of March 2007, in North America Firefox had a 15% share, and 25% in Europe.
The browser is officially supported in 41 languages, and is available for the different versions of Microsoft Windows, Apple Mac OS X, and Linux operating systems. Unofficial builds are available for Sun Microsystems Solaris, BSD and other operating systems.