diff --git a/services/crypto/tests/unit/head_helpers.js b/services/crypto/tests/unit/head_helpers.js
index 3b547cc37ae..b74e1f9ce03 100644
--- a/services/crypto/tests/unit/head_helpers.js
+++ b/services/crypto/tests/unit/head_helpers.js
@@ -13,9 +13,9 @@ catch(ex) {
 
 // Make sure to provide the right OS so crypto loads the right binaries
 var OS = "XPCShell";
-if ("@mozilla.org/windows-registry-key;1" in Cc)
+if (mozinfo.os == "win")
   OS = "WINNT";
-else if ("nsILocalFileMac" in Ci)
+else if (mozinfo.os == "mac")
   OS = "Darwin";
 else
   OS = "Linux";
diff --git a/services/sync/tests/unit/head_appinfo.js b/services/sync/tests/unit/head_appinfo.js
index 3929714735d..b99d4e0cb09 100644
--- a/services/sync/tests/unit/head_appinfo.js
+++ b/services/sync/tests/unit/head_appinfo.js
@@ -19,9 +19,9 @@ Services.prefs.setCharPref("identity.sync.tokenserver.uri", "http://token-server
 
 // Make sure to provide the right OS so crypto loads the right binaries
 var OS = "XPCShell";
-if ("@mozilla.org/windows-registry-key;1" in Cc)
+if (mozinfo.os == "win")
   OS = "WINNT";
-else if ("nsILocalFileMac" in Ci)
+else if (mozinfo.os == "mac")
   OS = "Darwin";
 else
   OS = "Linux";