If you get this error, it generally means firefox exited without removing it's lock files. You can do the following to fix is s
First, make sure you're not running firefox and kill them if you have any (note, there may be other users running firefox, this is not a problem and is irrelevant to this error:
ps -x | grep -i firefox killall firefox
Then look for any lock files and remove them.
cd $HOME find .mozilla/firefox -name "*lock" -ls
Use the rm command to remove the files.