If, when attempting to start firefox, you get an error that says something like:
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.
First, do a
ps -ef|grep firefox
command to make sure it isn’t. If it is, kill it with
kill -9 < pid >
If it’s not running, there are probably some lock files that need to be deleted. Look in your ~/.mozilla directory and search for lock and .parentlock. Then delete these files.