When you try to start Firefox sometimes it does not respond and gives a message
“Firefox is already running ,but it is not responding.To open a new window,you must close the existing Firefox Process,or restart your system .“
Reason for This:The reason for this, firefox puts a lock file to users profile folder when firefox starts and it removes the lock file from the profile folder when firefox is closed.Remember it uses only one profile at a time. When your lock file already exists when firefox starts then in that case firefox gives above error message.
Solution for this error:The solution is just open task manger by pressing Alt+Ctrl+Delete and select the firefox.exe process and click End Process