[caslon]

Icecat is a fork of Firefox to rid it of proprietary services and DRM. It's got a different name because Mozilla forbids (or used to forbid) redistribution with patches.

Debian also used to do this until Mozilla changed their tune on a few things. Debian's version and Icecat both used to be named IceWeasel, though, making fun of Mozilla.

[nitia]

Icecat is woefully out of date and contains a bunch of intrusive addons that break most sites, by default. YMMV.

[uncletaco]

Icecat tracks The latest ESR and is up to date.