There are several problems with using GNOME 3.28 and Icecat 60 on Guix, many of which impact each other.
1. The gnome-tweaks package can't find gnome-shell extensions from the gnome-shell-extensions package, and so claims none are installed. This is a packaging problem because on normal distributions, packaged extensions are visible to gnome-tweaks. 2. The stock "Web" browser is not able to install shell extensions. This may be an upstream bug. 3. Icecat can not install gnome-shell extensions because the Native Connector is not packaged in Guix. Despite the misleading name, this is the tool needed. https://wiki.gnome.org/Projects/GnomeShellIntegrationForChrome/Installation 4. Icecat is unable to sign in to Firefox sync. This is apparently fixed in 68.