override stopClient without params in FirefoxDriver

This enables us to ensure we call quit on the Legacy FirefoxDriver
And thus cleans up the temporary file system.

Fixes #2914
Fixes #2908

java/client/src/org/openqa/selenium/firefox/FirefoxDriver.java

@@ -384,8 +384,8 @@ protected Lock obtainLock(FirefoxProfile profile) {
   }
 
   @Override
-  protected void stopClient(Capabilities desiredCapabilities, Capabilities requiredCapabilities) {
-    if (isLegacy(desiredCapabilities)) {
+  protected void stopClient() {
+    if (this.getCommandExecutor() instanceof LazyCommandExecutor) {
       ((LazyCommandExecutor) this.getCommandExecutor()).quit();
     }
   }