diff --git a/flake.lock b/flake.lock
index a3ed534..1696d7f 100644
--- a/flake.lock
+++ b/flake.lock
@@ -17,6 +17,28 @@
         "type": "github"
       }
     },
+    "firefox-addons": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "dir": "pkgs/firefox-addons",
+        "lastModified": 1780977789,
+        "narHash": "sha256-UFJfQlvInbsVaTK5XC2lafdqWlwiNP5LuQFYfDKq6Dc=",
+        "owner": "rycee",
+        "repo": "nur-expressions",
+        "rev": "0b627f105ea3baa2fa10308a6a67a8f8cbbb3e2a",
+        "type": "gitlab"
+      },
+      "original": {
+        "dir": "pkgs/firefox-addons",
+        "owner": "rycee",
+        "repo": "nur-expressions",
+        "type": "gitlab"
+      }
+    },
     "flake-compat": {
       "locked": {
         "lastModified": 1761640442,
@@ -204,6 +226,7 @@
     },
     "root": {
       "inputs": {
+        "firefox-addons": "firefox-addons",
         "flake-parts": "flake-parts",
         "home-manager": "home-manager",
         "nix-darwin": "nix-darwin",
diff --git a/flake.nix b/flake.nix
index c860a3e..07ee130 100644
--- a/flake.nix
+++ b/flake.nix
@@ -23,6 +23,11 @@
     # Provides mkFlake: the systems/perSystem scaffolding used below.
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+    # Declarative Firefox add-ons (e.g. the Catppuccin theme); see lyrathorpe/user.nix.
+    firefox-addons = {
+      url = "gitlab:rycee/nur-expressions?dir=pkgs/firefox-addons";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =
diff --git a/lyrathorpe/home/desktop.nix b/lyrathorpe/home/desktop.nix
index 0bfddf4..da6f757 100644
--- a/lyrathorpe/home/desktop.nix
+++ b/lyrathorpe/home/desktop.nix
@@ -2,7 +2,13 @@
 # Imported only on hosts that run Sway (MBP, T400, Mac Pro); never pulled onto
 # the headless WSL host. Login (and the Sway session launch) is handled by the
 # greetd/ReGreet greeter -- see ../swaywm.nix -- so there is no tty1 autostart.
-{ pkgs, config, ... }:
+{
+  pkgs,
+  config,
+  inputs,
+  username,
+  ...
+}:
 {
   imports = [
     ./sway.nix
@@ -57,4 +63,31 @@
     name = "Adwaita";
     size = 24;
   };
+
+  # Firefox is themed at the browser level (it does not follow the GTK theme).
+  # The system installs the binary (programs.firefox in ../user.nix); here
+  # home-manager owns only the profile, hence package = null. Apply the
+  # Catppuccin Mocha theme add-on (only the mauve accent is packaged upstream;
+  # the rest of the desktop uses blue) and make content + UI dark.
+  programs.firefox = {
+    enable = true;
+    package = null;
+    profiles.${username} = {
+      id = 0;
+      isDefault = true;
+      extensions = {
+        force = true;
+        packages = [
+          inputs.firefox-addons.packages.${pkgs.system}.catppuccin-mocha-mauve
+        ];
+      };
+      settings = {
+        # Enable bundled add-ons automatically so the theme applies on first run.
+        "extensions.autoDisableScopes" = 0;
+        # Dark chrome + dark page content.
+        "ui.systemUsesDarkTheme" = 1;
+        "layout.css.prefers-color-scheme.content-override" = 0;
+      };
+    };
+  };
 }