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. This doesn't work because Firefox is not a running process so I can't close it.
I don't use Firefox now because of this running problem. It started about a month ago. So when I access the task manager, Firefox does not show up in the running processes. Because of this, I can't end the task. I got tired and fed up of rebooting my computer just to use Firefox, so I just switched to IE8, and Opera and Google Chrome. I prefered Firefox but it also runs so so sooooo slowly now. IE is much much faster, as are the other browsers now. Once upon a time, I bragged to everyone about how fast Firefox used to be, where now I admit I don't use it because it's so slow now and it won't run anyway.