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
