Firefox is already running but is not responding. You must either close Firefox or restart your system.
If you receive this type of error message when attempting to open Firefox, it is most likely referring to a previous (locked) process or session. In Linux, you can type the following commands to remove the lock file from your ~/.mozilla configuration folder, but you may need to examine the folder to determine the exact location of the lock file:
rm -f ~/.mozilla/firefox/*.default/lock
rm -f ~/.mozilla/firefox/*.default/.parentlock