{
  config,
  inputs,
  lib,
  pkgs,
  util,
  ...
}:
# Configures Firefox as the desktop browser.
let
  cfg = config.modules.desktop.apps.firefox;
in
{
  options.modules.desktop.apps.firefox = {
    enable = lib.mkEnableOption "Firefox as the desktop browser";

    uiScale = lib.mkOption {
      type = lib.types.str;
      default = "1.0";
      description = ''
        Scales Firefox UI and page content.

        Sets `layout.css.devPixelsPerPx`.
        Use a larger value for high-DPI displays.
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    registry.window-icon.${config.networking.hostName}.firefox = {
      class = ".*[Ff]irefox.*";
      icon = util.glyph "f269";
    };

    home-manager.users.${config.user.name} = hm: {
      programs.firefox = {
        enable = true;

        # Applies policies that users cannot change in Firefox.
        policies = {
          DisableTelemetry = true;
          DisableFirefoxStudies = true;
          DisablePocket = true;
          OfferToSaveLogins = false;
          DontCheckDefaultBrowser = true;
          DisableFirefoxAccounts = true;

          # Removes sponsored content from the new-tab page.
          FirefoxHome = {
            SponsoredTopSites = false;
            SponsoredPocket = false;
            Snippets = false;
          };
        };

        profiles.default = {
          extensions = {
            packages = with inputs.firefox-addons.packages.${pkgs.stdenv.hostPlatform.system}; [
              ublock-origin
              proton-pass
              sponsorblock
            ];

            # Home Manager overwrites Nord theme changes made in Firefox.
            force = true;
          };

          # Stylix uses a near-white selection background.
          # Use the Nord selection color because its text stays readable.
          extensions.settings."FirefoxColor@mozilla.com".settings.theme.colors.popup_highlight =
            let
              c = hm.config.lib.stylix.colors;
            in
            lib.mkForce {
              r = c."base03-rgb-r";
              g = c."base03-rgb-g";
              b = c."base03-rgb-b";
            };

          settings = {
            "layout.css.devPixelsPerPx" = cfg.uiScale;

            # Enables Firefox Color after Firefox sideloads the add-on.
            "extensions.autoDisableScopes" = 0;

            # Removes sponsored content outside Firefox policies.
            "browser.urlbar.suggest.quicksuggest.sponsored" = false;
            "browser.newtabpage.activity-stream.showSponsored" = false;
            "browser.newtabpage.activity-stream.showSponsoredTopSites" = false;
          };

          search = {
            # Home Manager replaces the engine list when this setting is enabled.
            # Engines added in Firefox do not survive a rebuild.
            force = true;
            default = "ddg";

            # Hides commercial search engines.
            # Bangs still reach these engines.
            # Entries with only `metaData` configure Firefox built-in engines.
            engines = {
              google.metaData.hidden = true;
              bing.metaData.hidden = true;
              ebay.metaData.hidden = true;
              "amazondotcom-us".metaData.hidden = true;
            };
            order = [ "ddg" ];
          };
        };
      };

      # Applies the Nord theme through Stylix's Firefox Color add-on.
      stylix.targets.firefox = {
        enable = true;
        profileNames = [ "default" ];
        colorTheme.enable = true;
      };

      # Links opened from other applications land in Firefox.
      xdg.mimeApps = {
        enable = true;
        defaultApplications = {
          "text/html" = "firefox.desktop";
          "application/xhtml+xml" = "firefox.desktop";
          "x-scheme-handler/http" = "firefox.desktop";
          "x-scheme-handler/https" = "firefox.desktop";
        };
      };

      # Firefox creates `profiles.ini` on its first launch.
      # Force replacement during Home Manager activation.
      home.file."${hm.config.programs.firefox.configPath}/profiles.ini".force = true;
    };
  };
}