diff --git a/pkgs/firefox.nix b/pkgs/firefox.nix
index 5ab18f0..0161155 100644
--- a/pkgs/firefox.nix
+++ b/pkgs/firefox.nix
@@ -24,6 +24,11 @@
           install_url = "https://addons.mozilla.org/firefox/downloads/latest/darkreader/latest.xpi";
           installation_mode = "force_installed";
         };
+        # Advent of Code delta-rank
+        "{78b07db2-25ed-4088-9c92-5c5dcfca3c3c}" = {
+          install_url = "https://addons.mozilla.org/firefox/downloads/latest/advent-of-code-delta-rank/latest.xpi";
+          installation_mode = "force_installed";
+        };
       };
     };
   };