It's an old issue (several years, at least), but it still happens. Ocasionally, the Mozilla Firefox browser will fail to start, citing the error: "Firefox is already running but not responding".
Other instances of the browser may of may not be running.
What you do, is pop up a terminal window, and type the following:
$ killall firefox$ rm -fr $HOME/.mozilla/firefox/*default/.parentlock
After the .parentlock lock file is gone, firefox should start as normal.
On shared machines with many novice users, I'd put the following in $HOME/.bashrc:
alias firefox='rm -fr $HOME/.mozilla/firefox/*default/.parentlock; firefox'That way, everytime you run firefox, the lock gets deleted first. In my experience, there aren't any noticable side effects. But do drop me a comment if you find any.
