--- a/toolkit/components/jsoncpp/src/lib_json/moz.build
+++ b/toolkit/components/jsoncpp/src/lib_json/moz.build
@@ -39,11 +39,15 @@ if CONFIG['_MSC_VER']:
 elif CONFIG['GNU_CXX']:
     CXXFLAGS += [
         '-Wno-unused-local-typedefs',
         '-Wno-shadow',
     ]
 
 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',
+    ]