Исключение selenium webdriver

Сегодня, когда я использую selenium webdrive, я получил ошибку. моя платформа - Mac OSX. это мой журнал исключений.

ruby-1.9.2-p0 > Selenium::WebDriver.for :firefox
Selenium::WebDriver::Error::WebDriverError: unable to bind to locking port 7054 within 45 seconds
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver/firefox/socket_lock.rb:48:in `lock'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver/firefox/socket_lock.rb:29:in `locked'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver/firefox/launcher.rb:32:in `launch'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver/firefox/bridge.rb:21:in `initialize'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver/common/driver.rb:38:in `new'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver/common/driver.rb:38:in `for'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/selenium-webdriver-0.1.0/lib/selenium/webdriver.rb:51:in `for'
 from (irb):8
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.1/lib/rails/commands/console.rb:44:in `start'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.1/lib/rails/commands/console.rb:8:in `start'
 from /Users/Apple/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.1/lib/rails/commands.rb:23:in `<top (required)>'
 from script/rails:6:in `require'
 from script/rails:6:in `<main>'

Я не знаю, почему это произошло. мой путь к firefox - путь по умолчанию. спасибо за вашу помощь !!