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 Firefox add-ons. Installing extensions this way will automatically enable extensions inside Firefox after the first installation.

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with pkgs.nur.repos.rycee.firefox-addons; [ privacy-badger { package = ublock-origin; settings = { private_browsing = true; }; } ]