--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -3173,16 +3173,28 @@ XREMain::XRE_mainInit(bool* aExitFlag)
 
   if (gfxPlatform::IsHeadless()) {
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK) || defined(XP_MACOSX)
     printf_stderr("*** You are running in headless mode.\n");
 #else
     Output(true, "Error: headless mode is not currently supported on this platform.\n");
     return 1;
 #endif
+
+#ifdef XP_MACOSX
+    // To avoid taking focus when running in headless mode immediately
+    // transition Firefox to a background application.
+    ProcessSerialNumber psn = { 0, kCurrentProcess };
+    OSStatus transformStatus = TransformProcessType(&psn, kProcessTransformToBackgroundApplication);
+    if (transformStatus != noErr) {
+      NS_ERROR("Failed to make process a background application.");
+      return 1;
+    }
+#endif
+
   }
 
   nsresult rv;
   ArgResult ar;
 
 #ifdef DEBUG
   if (PR_GetEnv("XRE_MAIN_BREAK"))
     NS_BREAK();