Are you using Windows? You don't say, but if this is a Mac, Linux, or anything else, I will pass.
If Windows, open up your Task Manager, (right click the Taskbar, select Task Manager), open the Processes tab, find and highlight Firefox.exe and click the "End Process" button.
It happens sometimes, especially if you visit web sites that a particular browser doesn't handle well.
when I tey to open a window
this message comes on
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.
but I have no window open, why does this message come on ?