when i exit firefox &try to reopen the message is showing as 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.why this happens even i close it by exit.

after i exit firefox & tried to open,it is not opening,instead this message is coming as '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.'kindly help me.everytime to open i have to go to task manager & stop the process.