diff --git a/src/main/java/io/vertx/core/http/impl/HttpUtils.java b/src/main/java/io/vertx/core/http/impl/HttpUtils.java
index 259f961c6ec..f147e875476 100644
--- a/src/main/java/io/vertx/core/http/impl/HttpUtils.java
+++ b/src/main/java/io/vertx/core/http/impl/HttpUtils.java
@@ -1008,7 +1008,19 @@ static void resolveFile(VertxInternal vertx, String filename, long offset, long
   }
 
   static boolean isConnectOrUpgrade(io.vertx.core.http.HttpMethod method, MultiMap headers) {
-    return method == io.vertx.core.http.HttpMethod.CONNECT || (method == io.vertx.core.http.HttpMethod.GET && headers.contains(io.vertx.core.http.HttpHeaders.CONNECTION, io.vertx.core.http.HttpHeaders.UPGRADE, true));
+    if (method == io.vertx.core.http.HttpMethod.CONNECT) {
+      return true;
+    }
+    if (method == io.vertx.core.http.HttpMethod.GET) {
+      for (String connection : headers.getAll(io.vertx.core.http.HttpHeaders.CONNECTION)) {
+        // Firefox doesn't send a normal 'Connection: Upgrade' header for WebSockets.
+        // Instead, it sends: 'Connection: keep-alive, Upgrade'.
+        if (AsciiString.containsIgnoreCase(connection, io.vertx.core.http.HttpHeaders.UPGRADE)) {
+          return true;
+        }
+      }
+    }
+    return false;
   }
 
   static boolean isKeepAlive(HttpRequest request) {
@@ -1031,9 +1043,9 @@ public static boolean canUpgradeToWebSocket(HttpServerRequest req) {
     }
     MultiMap headers = req.headers();
     for (String connection : headers.getAll(io.vertx.core.http.HttpHeaders.CONNECTION)) {
-      if (connection.toLowerCase().contains(io.vertx.core.http.HttpHeaders.UPGRADE)) {
+      if (AsciiString.containsIgnoreCase(connection, io.vertx.core.http.HttpHeaders.UPGRADE)) {
         for (String upgrade : headers.getAll(io.vertx.core.http.HttpHeaders.UPGRADE)) {
-          if (upgrade.toLowerCase().contains(io.vertx.core.http.HttpHeaders.WEBSOCKET)) {
+          if (AsciiString.containsIgnoreCase(upgrade, io.vertx.core.http.HttpHeaders.WEBSOCKET)) {
             return true;
           }
         }
diff --git a/src/test/java/io/vertx/core/http/Http1xTest.java b/src/test/java/io/vertx/core/http/Http1xTest.java
index 5fa5b261c03..212648ed0e8 100644
--- a/src/test/java/io/vertx/core/http/Http1xTest.java
+++ b/src/test/java/io/vertx/core/http/Http1xTest.java
@@ -25,7 +25,6 @@
 import io.vertx.core.impl.ContextInternal;
 import io.vertx.core.impl.Utils;
 import io.vertx.core.impl.VertxInternal;
-import io.vertx.core.impl.transports.JDKTransport;
 import io.vertx.core.json.JsonArray;
 import io.vertx.core.json.JsonObject;
 import io.vertx.core.net.*;
@@ -5351,16 +5350,31 @@ public void testCannotUpgradeToWebSocketIfUpgradeDoesNotContainWebsocket() throw
 
   private void doTestCanUpgradeToWebSocket(UnaryOperator<RequestOptions> config, boolean shouldSucceed) throws Exception {
     server.requestHandler(req -> {
-      HttpServerResponse serverResponse = req.response();
-      int statusCode = HttpUtils.canUpgradeToWebSocket(req) ? 200 : 500;
-      serverResponse.setStatusCode(statusCode);
-      serverResponse.end();
+      HttpServerResponse resp = req.response();
+      if (HttpUtils.canUpgradeToWebSocket(req)) {
+        resp.headers()
+          .set(HttpHeaders.CONNECTION, HttpHeaders.UPGRADE)
+          .set(HttpHeaders.UPGRADE, HttpHeaders.WEBSOCKET);
+        req.toNetSocket().onComplete(onSuccess(sock -> {
+          sock.write(Buffer.buffer("foo"));
+        }));
+      } else {
+        resp.setStatusCode(500).end();
+      }
     });
     startServer(testAddress);
     client.request(config.apply(new RequestOptions(requestOptions))).onComplete(onSuccess(req -> {
       req.response().onComplete(onSuccess(resp -> {
-        assertEquals(shouldSucceed ? 200 : 500, resp.statusCode());
-        testComplete();
+        if (shouldSucceed) {
+          assertEquals(101, resp.statusCode());
+          resp.netSocket().handler(buffer -> {
+            assertEquals("foo", buffer.toString());
+            testComplete();
+          });
+        } else {
+          assertEquals(500, resp.statusCode());
+          testComplete();
+        }
       }));
       req.send();
     }));