Selenium, PHPUnit and xvfb setup on ubuntu always uses firefox

Question

I've set up selenium, PHPUnit and xvfb on my ubuntu server. This is my TestCase:

class WebTest extends PHPUnit_Extensions_SeleniumTestCase {

   public static $browsers = array(
       array(
           'name' => 'Firefox on Linux',
           'browser' => '*firefox',
       ),
       array(
           'name' => 'Chrome',
           'browser' => '*chrome',
       ),
    );

   protected function setUp() {

      $this->setBrowserUrl("http://www.whatismybrowser.com/");
   }

   public function testTitle() {
      $this->open("/");
      try {
          $this->assertTrue((bool) preg_match('/^Firefox[\s\S]*$/', $this->getText("css=div.detected.simple-browser-string")));
      } catch (PHPUnit_Framework_AssertionFailedError $e) {
          array_push($this->verificationErrors, $e->toString());
      }
   }

This test calls http://www.whatismybrowser.com/ and should fail if http://www.whatismybrowser.com/ determines anything besides a firefox browser. However it passes two times. In spite of 'browser' => '*chrome', firefox is always used.

Do you have any ideas how to change this behavior?