diff --git a/tests/modules/programs/firefox/default.nix b/tests/modules/programs/firefox/default.nix
index cc8e2fc4..1df80c5b 100644
--- a/tests/modules/programs/firefox/default.nix
+++ b/tests/modules/programs/firefox/default.nix
@@ -2,4 +2,5 @@
   firefox-profile-settings = ./profile-settings.nix;
   firefox-state-version-19_09 = ./state-version-19_09.nix;
   firefox-deprecated-native-messenger = ./deprecated-native-messenger.nix;
+  firefox-duplicate-profile-ids = ./duplicate-profile-ids.nix;
 }
diff --git a/tests/modules/programs/firefox/duplicate-profile-ids.nix b/tests/modules/programs/firefox/duplicate-profile-ids.nix
new file mode 100644
index 00000000..41c09436
--- /dev/null
+++ b/tests/modules/programs/firefox/duplicate-profile-ids.nix
@@ -0,0 +1,23 @@
+{ config, lib, ... }:
+
+{
+  imports = [ ./setup-firefox-mock-overlay.nix ];
+
+  config = lib.mkIf config.test.enableBig {
+    test.asserts.assertions.expected = [''
+      Must not have Firefox profiles with duplicate IDs but
+        - ID 1 is used by first, second''];
+
+    programs.firefox = {
+      enable = true;
+
+      profiles = {
+        first = {
+          isDefault = true;
+          id = 1;
+        };
+        second = { id = 1; };
+      };
+    };
+  };
+}