diff --git a/tests/modules/programs/thunderbird/default.nix b/tests/modules/programs/thunderbird/default.nix
index 487297784..a17755e16 100644
--- a/tests/modules/programs/thunderbird/default.nix
+++ b/tests/modules/programs/thunderbird/default.nix
@@ -1 +1,4 @@
-{ thunderbird = ./thunderbird.nix; }
+{
+  thunderbird = ./thunderbird.nix;
+  thunderbird-with-firefox = ./thunderbird-with-firefox.nix;
+}
diff --git a/tests/modules/programs/thunderbird/thunderbird-with-firefox.nix b/tests/modules/programs/thunderbird/thunderbird-with-firefox.nix
new file mode 100644
index 000000000..bd1744fa8
--- /dev/null
+++ b/tests/modules/programs/thunderbird/thunderbird-with-firefox.nix
@@ -0,0 +1,5 @@
+# Confirm that both Firefox and Thunderbird can be configured at the same time.
+{ lib, realPkgs, ... }:
+lib.recursiveUpdate (import ./thunderbird.nix { inherit lib realPkgs; }) {
+  programs.firefox.enable = true;
+}
diff --git a/tests/modules/programs/thunderbird/thunderbird.nix b/tests/modules/programs/thunderbird/thunderbird.nix
index 555ea35d6..aa46b3b2d 100644
--- a/tests/modules/programs/thunderbird/thunderbird.nix
+++ b/tests/modules/programs/thunderbird/thunderbird.nix
@@ -37,9 +37,6 @@
     };
   };
 
-  # Confirm that both Firefox and Thunderbird can be configured at the same time.
-  programs.firefox = { enable = true; };
-
   programs.thunderbird = {
     enable = true;