Fix HMR in Firefox when proxy option present (#7444)

Resolves #6720

Author: dmile

packages/react-dev-utils/WebpackDevServerUtils.js

@@ -361,10 +361,14 @@ function prepareProxy(proxy, appPublicFolder) {
     process.exit(1);
   }
 
-  // If proxy is specified, let it handle any request except for files in the public folder.
+  // If proxy is specified, let it handle any request except for
+  // files in the public folder and requests to the WebpackDevServer socket endpoint.
+  // https://github.com/facebook/create-react-app/issues/6720
   function mayProxy(pathname) {
     const maybePublicPath = path.resolve(appPublicFolder, pathname.slice(1));
-    return !fs.existsSync(maybePublicPath);
+    const isPublicFileRequest = fs.existsSync(maybePublicPath);
+    const isWdsEndpointRequest = pathname.startsWith('/sockjs-node'); // used by webpackHotDevClient
+    return !(isPublicFileRequest || isWdsEndpointRequest);
   }
 
   if (!/^http(s)?:\/\//.test(proxy)) {