diff --git a/toolkit/components/jsoncpp/src/lib_json/moz.build b/toolkit/components/jsoncpp/src/lib_json/moz.build
index 752b590ff141..3bda236bd5e2 100644
--- a/toolkit/components/jsoncpp/src/lib_json/moz.build
+++ b/toolkit/components/jsoncpp/src/lib_json/moz.build
@@ -44,10 +44,6 @@ elif CONFIG['GNU_CXX']:
 
 if CONFIG['CLANG_CXX'] or CONFIG['CLANG_CL']:
     CXXFLAGS += [
+        '-Wno-implicit-fallthrough',
         '-Wno-c++11-narrowing',
     ]
-
-if CONFIG['CLANG_CXX'] or CONFIG['CLANG_CL'] or CONFIG['GNU_CXX']:
-    CXXFLAGS += [
-        '-Wno-implicit-fallthrough',
-    ]