GNUzilla is the GNU version of the Mozilla suite, and GNU IceCat is the GNU version of the Firefox browser. Its main advantage is an ethical one: it is entirely free software. While the Firefox source code from the Mozilla project is free software, they distribute and recommend non-free software as plug-ins and addons.
In addition, GNU IceCat includes some privacy protection features, included in a separate addon:
Some sites refer to zero-size images on other hosts to keep track of cookies. When IceCat detects this mechanism it blocks cookies from the site hosting the zero-length image file. (It is possible to re-enable such a site by removing it from the blocked hosts list.)
Other sites rewrite the host name in links redirecting the user to another site, mainly to “spy” on clicks. When this behavior is detected, IceCat shows a message alerting the user.
Origin of the name
The name “IceCat” was coined to show our relationship to the Mozilla Firefox browser. Ice isn’t Fire and a Cat isn’t a Fox, so it is clearly a different package, but is equally clearly intimately related (of course nearly all of the work comes from the Mozilla foundation effort).
The gNewSense BurningDog browser and the Debian IceWeasel browser are similarly derived from Firefox, also with the intent of being free software. Technically, however, these projects are maintained entirely independently of IceCat. (Previously, this GNU browser project was also named IceWeasel, but that proved confusing.)