GNU IceCat is the GNU version of the Firefox ESR browser. Extensions included to this version of IceCat:
HTTPS Everywhere is an extension that encrypts your communications with many major websites, making your browsing more secure.
Watch videos from video sharing websites with extra options.
A series of configuration changes and tweaks were applied to ensure that IceCat does not initiate network connections that the user has not explicitly requested. This implies not downloading feeds, updates, blacklists or any other similar data needed during startup.
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 (we don't want Mozilla blamed for our mistakes, nor cause confusion with their trademarks), but is equally clearly intimately related (of course nearly all of the work comes from the Mozilla foundation effort, so we want to give credit).
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.) About GNU and the GNU Philosophy
The free software philosophy is the root and motivation of the guidelines and goals of the whole free software movement, a worldwide community.