selenimu 在FF,及chrome运行报错，IE下运行正常?报错

 public static void main(String[] args)
  {
   String host = "localhost";
   int port = 4444;
   String url = "http://www.baidu.com/";
   String browserType = "*firefox";
 
   String keyWordsLocator = "document.getElementById('kw')";  
   String search = "document.getElementById('su')";
   DefaultSelenium selenium = new DefaultSelenium(host,port,browserType,url);
   selenium.start();
   selenium.open(url);
   selenium.type(keyWordsLocator,"java selenium");
   selenium.click(search);
   selenium.waitForPageToLoad("50000");
   selenium.stop();
  System.out.println("..............");
  }

browserType = "*firefox"; 改成*chrome也不行，改成iexplore则正常，报错如下:

Exception in thread "main" java.lang.RuntimeException: Could not start Selenium session: Failed to start new browser session: Error while launching browser
 at com.thoughtworks.selenium.DefaultSelenium.start(DefaultSelenium.java:109)
 at SeleniumDemo01.main(SeleniumDemo01.java:15)
Caused by: com.thoughtworks.selenium.SeleniumException: Failed to start new browser session: Error while launching browser
 at com.thoughtworks.selenium.HttpCommandProcessor.throwAssertionFailureExceptionOrError(HttpCommandProcessor.java:112)
 at com.thoughtworks.selenium.HttpCommandProcessor.doCommand(HttpCommandProcessor.java:106)
 at com.thoughtworks.selenium.HttpCommandProcessor.getString(HttpCommandProcessor.java:275)
 at com.thoughtworks.selenium.HttpCommandProcessor.start(HttpCommandProcessor.java:237)
 at com.thoughtworks.selenium.DefaultSelenium.start(DefaultSelenium.java:100)
 ... 1 more