当我尝试使用 FF headless 通过以下方式截取网站截图时：
firefox -screenshot google.com
FF 拒绝运行并显示以下错误消息：
*** You are running in headless mode.
Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.
我正在运行 FF 58.0.2。
我该如何解决？