Chen Yu -

gnuzilla no longer publishes installer packages - since v68,
icecat is maintained only as source code

the preferred way to install icecat is from the repos of a distro
which packages it - if you are using an arch-like distro, you
could use the icecat package from parabola - fedora users
(and other RPM-based distros probably) can use the fedora
package - for debian-based distros, there is a user-maintained
repo on the OBS - users of other distros can install the guix
package manager and use that to install icecat

of course, anyone could build it from source - that is done by
running the makeicecat script, which cleans the mozilla
sources, then running the ./mach script to compile in the usual
mozilla way - there is probably a README that explains the
procedure in detail