diff --git a/toolkit/components/startup/moz.build b/toolkit/components/startup/moz.build
index cf3489345959..8aa291e68434 100644
--- a/toolkit/components/startup/moz.build
+++ b/toolkit/components/startup/moz.build
@@ -14,7 +14,7 @@ EXPORTS.mozilla += [
 
 XPCSHELL_TESTS_MANIFESTS += ['tests/unit/xpcshell.ini']
 
-SOURCES += [
+UNIFIED_SOURCES += [
     'nsAppStartup.cpp',
     'StartupTimeline.cpp',
 ]
@@ -24,15 +24,16 @@ if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'os2':
         'nsUserInfoOS2.cpp',
     ]
 elif CONFIG['OS_ARCH'] == 'WINNT':
+    # This file cannot be built in unified mode because of name clashes with Windows headers.
     SOURCES += [
         'nsUserInfoWin.cpp',
     ]
 elif CONFIG['MOZ_WIDGET_TOOLKIT'] == 'cocoa':
-    SOURCES += [
+    UNIFIED_SOURCES += [
         'nsUserInfoMac.mm',
     ]
 else:
-    SOURCES += [
+    UNIFIED_SOURCES += [
         'nsUserInfoUnix.cpp',
     ]