--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -472,21 +472,20 @@ Preferences::IsServiceAvailable()
 {
   return !!sPreferences;
 }
 
 // static
 bool
 Preferences::InitStaticMembers()
 {
+  if (!sShutdown && !sPreferences) {
 #ifndef MOZ_B2G
-  MOZ_ASSERT(NS_IsMainThread());
+    MOZ_ASSERT(NS_IsMainThread());
 #endif
-
-  if (!sShutdown && !sPreferences) {
     nsCOMPtr<nsIPrefService> prefService =
       do_GetService(NS_PREFSERVICE_CONTRACTID);
   }
 
   return sPreferences != nullptr;
 }
 
 // static