Change devtools install message on file: URLs

src/renderers/dom/ReactDOM.js

@@ -78,8 +78,13 @@ if (__DEV__) {
       if ((navigator.userAgent.indexOf('Chrome') > -1 &&
           navigator.userAgent.indexOf('Edge') === -1) ||
           navigator.userAgent.indexOf('Firefox') > -1) {
+        // Firefox does not have the issue with devtools loaded over file://
+        var showFileUrlMessage = window.location.protocol.indexOf('http') === -1 &&
+          navigator.userAgent.indexOf('Firefox') === -1;
         console.debug(
-          'Download the React DevTools for a better development experience: ' +
+          'Download the React DevTools ' +
+          (showFileUrlMessage ? 'and use an HTTP server (instead of a file: URL) ' : '') +
+          'for a better development experience: ' +
           'https://fb.me/react-devtools'
         );
       }