Add-on package to install under policies. For a package to work here it needs and addonId exposed in it's passthru. This will be included in all add-ons accessible from the Nix User Repository. Once you have NUR installed run
$ nix-env -f '<nixpkgs>' -qaP -A nur.repos.rycee.firefox-addons
to list the available LibreWolf add-ons.
Installing extensions this way will automatically enable extensions inside LibreWolf after the first installation.