Iceweasel is Parabola's adaptation of the Mozilla Firefox web browser, originally re-branded by the Debian project. It is a powerful, extensible web browser with support for modern web application technologies.
Iceweasel differs technically from Firefox, mainly in that it removes support for DRM and other features that the FSDG considers to be non-free or anti-features, such as DRM and recommendations of non-free add-ons.
These are essentially the same differences as Guzilla IceCat; though IceCat is based on the LTS release-cycle of Firefox-ESR, where IceWeasel is based on the rolling release-cycle of Firefox (see Rolling_vs._LTS).
See the IceWeasel_History article for more information.