GNU IceCat, formerly known as GNU IceWeasel,[3] is a free software rebranding of the Mozilla Firefox web browser distributed by the GNU Project. It is compatible with both the GNU/Linux and Mac OS X 10.4 and 10.5 operating systems.The GNU Project keeps IceCat in synchronization with upstream development of Firefox while removing all trademarked artwork. It also maintains a large list of free software plugins and add-ons, and includes a few security features not found in the mainline Firefox browser.