Firefox is already running，实际后台查不到进程了

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.”

解决方法如下：
在linux的终端输入：firefox -profilemanager

回车后会出现一个小窗口让你选择使用哪个 profile 来启动 firefox。

首先将当前出错的Profile删除掉，然后新建下即可。