Selenium与PHP firefox浏览器问题

我在使用Firefox 35版本的ubuntu系统上运行selenium-server-standalone-2.42.2.jar。我正在使用phpwebdriver。当我运行我的Selenium脚本时，它将打开Firefox浏览器并在此处结束。我的脚本是：

<?php
require_once "phpwebdriver/WebDriver.php";

$webdriver = new WebDriver("localhost", "4444");
$webdriver->connect("firefox");                            
$webdriver->get("http://google.com");
$element = $webdriver->findElementBy(LocatorStrategy::name, "q");
if ($element) {
    $element->sendKeys(array("php webdriver" ) );
    $element->submit();
}

//$webdriver->close();

?>

当我运行此脚本时，它将打开Firefox并在此处停止，最后它会引发此错误。我不知道该怎么解决。

stdClass Object
(
    [status] => 13
    [sessionId] => 
    [value] => stdClass Object
        (
            [message] => POST /session//element
Build info: version: '2.42.2', revision: '6a6995d', time: '2014-06-03 17:42:03'
System info: host: 'zaptech', ip: '127.0.1.1', os.name: 'Linux', os.arch: 'amd64', os.version: '4.2.0-27-generic', java.version: '1.7.0_121'
Driver info: driver.version: unknown
            [localizedMessage] => POST /session//element
Build info: version: '2.42.2', revision: '6a6995d', time: '2014-06-03 17:42:03'
System info: host: 'zaptech', ip: '127.0.1.1', os.name: 'Linux', os.arch: 'amd64', os.version: '4.2.0-27-generic', java.version: '1.7.0_121'
Driver info: driver.version: unknown
            [suppressed] => Array
                (
                )

            [cause] => 
            [supportUrl] => 
            [systemInformation] => System info: host: 'zaptech', ip: '127.0.1.1', os.name: 'Linux', os.arch: 'amd64', os.version: '4.2.0-27-generic', java.version: '1.7.0_121'
            [class] => org.openqa.selenium.UnsupportedCommandException
            [additionalInformation] => 
Driver info: driver.version: unknown
            [hCode] => 1436132011
            [stackTrace] => Array
                (
                    [0] => 
                    [1] => 
                    [2] => 
                    [3] => 
                    [4] => 
                    [5] => 
                    [6] => 
                    [7] => 
                    [8] => 
                    [9] => 
                    [10] => 
                    [11] => 
                    [12] => 
                    [13] => 
                    [14] => 
                    [15] => 
                    [16] => 
                    [17] => 
                    [18] => 
                )

        )

    [state] => unhandled error
    [class] => org.openqa.selenium.remote.Response
    [hCode] => 1502000244
)
PHP Fatal error:  Uncaught exception 'WebDriverException' with message '13' in /var/www/html/selenium/wdtest/phpwebdriver/WebDriverBase.php:130
Stack trace:
#0 /var/www/html/selenium/wdtest/phpwebdriver/WebDriverBase.php(170): WebDriverBase->handleResponse(Object(stdClass))
#1 /var/www/html/selenium/wdtest/index.php(24): WebDriverBase->findElementBy('name', 'q')
#2 {main}
  thrown in /var/www/html/selenium/wdtest/phpwebdriver/WebDriverBase.php on line 130

我想念什么？还是浏览器兼容性问题？