GNU IceCat, formerly known as GNU IceWeasel, 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.

IceCat includes additional privacy and security features, such as the option to block third party zero-length image files resulting in third party cookie, also known as web bugs (This feature is available in Firefox 1.0, 1.5, and 3.0, but the UI option was absent on 2.0). GNU IceCat also provides warnings for URL redirection.

In version 3.0.2-g1, the certificate of CAcert.org, a certificate authority, has been added to the list of trusted root certificates. Concern about that decision has been raised in a discussion on the savannah-hackers-public mailing list.