The Fork of Firefox.
Icecat is based on Mozilla Firefox ESR (Extended Support Release).
When Mozilla releases a new version of Firefox ESR, a new source
tarball is made available
via their FTP.
Then with the script ‘makeicecat‘
the source code of Firefox is modified into
Icecat source code.
As you can read from the Gnu website about
Icecat:
The IceCat project does not currently
distribute binaries.
For this reason I decided to bridge this gap and build Icecat for as
many Operating Systems as possible. I work in order to make the newest
version available for all Operating Systems over time,
promptly following the official FIrefox ESR releases.