Mozilla Firefox is a free and open source Gecko-based web browser. It is the current default for the Ubuntu desktop.
"The Firefox browser is created by Mozilla, a non-profit organization whose mission is to promote openness, innovation and opportunity on the Web" (http://www.mozilla.org/en-US/firefox/about). Used by about 33% of the global browser market (W3C statistics), it is available for Windows, Mac OS X, and Linux, and is the default web browser on Ubuntu.
New versions of Firefox are currently developed and released on six weeks schedule, which Mozilla calls rapid release cycle. Mozilla also maintains Extended Support Releases (ESR).