firefox: set non-default configPath on darwin

On macOS 27+, wrapped package will have no Mozilla signature and will
fail to access the default datadir. Move users with recent stateVersion
to a non-default datadir recommended by:

https://github.com/NixOS/nixpkgs/pull/556611

Users with existing profiles under default datadir will have to migrate
them to a different location on macOS 27 upgrade. Migrating existing
directory is out of scope for Home Manager.

tests/modules/programs/firefox/config-path-darwin-default.nix

@@ -0,0 +1,74 @@
+{
+  expectedDarwinPath,
+  packageIsNull ? false,
+  stateVersion,
+}:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.programs.firefox;
+
+  expectedConfigPath =
+    if pkgs.stdenv.hostPlatform.isDarwin then expectedDarwinPath else ".config/mozilla/firefox";
+
+  expectedAppDataDir =
+    if
+      pkgs.stdenv.hostPlatform.isDarwin && !packageIsNull && lib.versionAtLeast stateVersion "26.11"
+    then
+      "${config.home.homeDirectory}/${expectedDarwinPath}"
+    else
+      null;
+
+  firefoxPackage = lib.makeOverridable (
+    {
+      cfg ? { },
+      extraPolicies ? { },
+      pkcs11Modules ? [ ],
+      appDataDir ? null,
+    }:
+    builtins.deepSeq
+      [
+        cfg
+        extraPolicies
+        pkcs11Modules
+      ]
+      (
+        config.lib.test.mkStubPackage {
+          name = "firefox-config-path-darwin-default-test-stub";
+          extraAttrs = {
+            browserName = "firefox";
+            inherit appDataDir;
+            meta.mainProgram = "firefox";
+          };
+        }
+      )
+  ) { };
+in
+{
+  config = lib.mkIf config.test.enableBig {
+    home.stateVersion = stateVersion;
+
+    programs.firefox = {
+      enable = true;
+      package = if packageIsNull then null else firefoxPackage;
+      profiles.test.settings."general.smoothScroll" = false;
+    };
+
+    assertions = [
+      {
+        assertion = cfg.configPath == expectedConfigPath;
+        message = "Firefox configPath has an unexpected default";
+      }
+      {
+        assertion = cfg.finalPackage == null || cfg.finalPackage.appDataDir == expectedAppDataDir;
+        message = "Firefox wrapper received an unexpected appDataDir";
+      }
+    ];
+
+    mozilla.firefoxNativeMessagingHosts = lib.mkForce [ ];
+  };
+}

tests/modules/programs/firefox/default.nix

@@ -4,6 +4,19 @@
   "firefox-bookmarks-legacy-attrset-warning" = ./bookmarks-legacy-attrset-warning.nix;
   "firefox-config-path-explicit-legacy" = ./config-path-explicit-legacy.nix;
   "firefox-config-path-explicit-xdg" = ./config-path-explicit-xdg.nix;
+  "firefox-config-path-darwin-default-current" = import ./config-path-darwin-default.nix {
+    expectedDarwinPath = "Library/Application Support/org.nixos.firefox";
+    stateVersion = "26.11";
+  };
+  "firefox-config-path-darwin-default-legacy" = import ./config-path-darwin-default.nix {
+    expectedDarwinPath = "Library/Application Support/Firefox";
+    stateVersion = "26.05";
+  };
+  "firefox-config-path-darwin-default-null-package" = import ./config-path-darwin-default.nix {
+    expectedDarwinPath = "Library/Application Support/Firefox";
+    packageIsNull = true;
+    stateVersion = "26.11";
+  };
   "firefox-config-path-wrapper-absolute" = import ./config-path-wrapper.nix {
     absolute = true;
     configPath = "Library/Application Support/org.nixos.firefox";