Firefox is not free libre software. This is why it doesn't ship on free Gnu/Linux distributions, they prefer alternatives such as Iceweasel or Chromium instead (free libre versions of Firefox and Chrome that don't come with any proprietary logos, mpg4 codecs, pdf readers etc).
Iceweasel is not really much of a fork, just a s/Firefox/Iceweasel/g over every file in the project because Debian considers Mozilla's branding requirements to conflict with their free software guidelines (DFSG).
