Unfortunately, upstream IceCat appears to use the FireFox 60.x ESR branch, which is three branches behind the current ESR and is past its end-of-life date. It's also a few security updates behind the 60.x ESR branch's last release.

The GUIX package is more recent, but its description states:

> WARNING: IceCat 91 has not yet been released by the upstream IceCat project. This is a preview release, and does not currently meet the privacy-respecting standards of the IceCat project.