Current Firefox releases use the time stamp of the parent.lock file to check for multiple startup failures. * Auto detect previous start-up failure and offer to start in safe mode (bug 294260) There is usually no problem with the presence of the parent.lock file unless Firefox is still running for some reason. Did you check in the Windows Task Manager on the Processes tab that all Firefox processes have been finished? Firefox 30, to be released on Tuesday, will include a fix caused by using "Clear history when Firefox closes" to clear the history. See also: *"Hang at exit": http://kb.mozillazine.org/Firefox_hangs *"Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs